Concepts
The most important part of a scenario's design is the initial concept. Will it be a historical campaign, an empire builder, a dungeons and dragons style adventure or an epic space opera? What will the victory objective be and who is the protagonist?
Get it right and you can be onto a winner, make critical errors in judgement early on in a project and you will find yourself struggling to get around them and your plans may ultimately fail!
As a general rule you should think about the strengths and weaknesses of the Civ2 game engine. Will your scenario idea work with the AI and does it play to the strengths of the game?
A lot can be achieved with events and clever scenario design but some scenarios suit the Civ2 engine better than others. For example World War 2 scenarios with fast moving offensives lend themselves well to Civ2 whilst the trenches and stalemate of World War One are hard to replicate. That's not to say that it can't be done, but it will require a lot of work and careful planning.
Think carefully about what you want to achieve with your scenario. If it is your first attempt why not try something modest in scale. Get it working well and looking good. You will build up experience and can increase the scope with your next project. I know from personal experience how tempting it is to try and build the biggest and best scenario possible from the off, but in hindsight I would have been better of setting myself more realistic goals. We all learn by experience and even a failed scenario can be a great learning experience, but I can tell you that there are few things more satisfying for a designer than releasing a fully functioning piece of work and hearing positive feedback from the community.
Map Making
There are several options. You can use an existing map if one exists (fingers crossed), use a utility to make a new map or build one by hand (my personal preference).
I work out the shape of the map first by making a scan or photocopy and drawing a box around the area you want to make. Then you need to use the map editor to make the map on there the right height x length. This may take a little playing around to get right. Next draw a grid this map using tundra or whatever (4x4 or 5x5 squares work well). Add up the number of squares on each axis then measure your paper map dimensions. Divide the dimensions by the number of squares (IE 120cm divided by 30 squares = 4cm per square on the paper map). Draw the grid on the paper map and check it matches the one on the editor. Start copying across square by square. Once you have the shape of the landmass set out add rivers, then mountains and so on. Remember to plot the locations of key cities before you blend out the grid. That's how I do things anyway. Its slow but it produces very accurate looking maps.
I draw a grid using paint or some such program. I use a horizontal/vertical lined map as it is a guide rather than a square by square plan. Getting the map the right size may take a bit of playing around as I think it needs to be twice as high as it is long (or it it the other way?).
The image I used with a grid drawn on with MS Paint:
How the map looks on the map editor after removing the corresponding grid:
Frustrating Agricola
See
this entertaining and informative article by Techumseh on how to design the toughest possible scenario to challenge the best players. Our dear friend Agricola sadly passed away but his legendary play-testing skills and witty feedback reports will live long in the memory of the Scenario League!
Special Terrains (+Food/Shields/Trade/Defence/Minefields)
Sometimes getting the resources for key cities right can be fiddly with the vanilla terrains. A quick and easy solution is to make use of the extra ToTPP terrain slots to create special terrains (such as high shield, food or trade output). This way large industrial cities can easily be given the output they require with the addition of a couple of high shield terrains. This is not a new concept, but ToTPP has made the process much easier.
Other special terrains I have used are high defence villages which I can place at strategic locations (you could use trenches, riverbed or whatever). One cool technique I have borrowed is the Minefield from Techumseh's Operation Market Garden scenario. This terrain type is impassable for most units (except engineers) who can move into the square and transform it (clear the mines) allowing other units to pass.
Spread of defensive terrain
One of the things I really enjoy when playing a scenario is making the best use of terrain, both for defence and attack. A flat, featureless map is often not that much fun to play, unless it is a deliberate design feature. Try and mix up the terrain as much as possible so that units can be deployed outside of cites in defensive positions. Where's the fun in just moving huge siege trains from one city to the next?
With the extra ToTPP terrains it is more feasible than ever to include unique terrains, like villages with high defence values, impassable minefields or rivers that act as a barrier to attacking troops. You can also include large rivers (ocean terrain) to create crossing points and funnel the action along historical lines.
John Ellis's farming system
One of the unique features of John Ellis' classic Seize the Crown was the system of agriculture. Instead of basic plains and grassland that could be irrigated John introduced a whole system where fields needed to be prepared (engineer transformed) before they could be irrigated or changed into pasture of grazing animals. I forget the exact details right now but I would recommend checking the scenario out for inspiration.
Curtsiblings Oilfield system
I don't know for sure if this is an original idea or borrowed from elsewhere, but I first noticed it in Curtsibling's Imperialism 1870 for ToTPP update. The normally fairly unproductive swamp terrain can be converted by engineers to Oil Fields once the prerequisite technology is obtained. The oldfield terrain gives a much higher shield output that can be further increased by building a mine on the square. This is a really simple but very effective way to represent the importance of oil in the industrial period. Think of Arabia prior to the 20th century. The seemingly barren deserts suddenly spouting forth black gold. Of course this concept can be adapted to many other concepts.
Tech Tree
Will you design a traditional tech tree that mimics the original Civ2 vanilla game or use events in a completely different way. For scenarios that cover a specific conflict you might consider using the events as a way of producing reinforcements via events.
Events
Events are the most powerful tool at a scenario designers disposal and can transform a most scenarios from average to outstanding. Take a look at the events.txt file of other scenarios for ideas. You can always copy & then edit other peoples events into your own events folder. No need to write them from scratch.
Fine Tuning
This is such a crucial aspect of scenario designing. For me there's nothing worse than starting a scenario to find loads of unnecessary popups (City already producing, civil unrest in etc...) that are not explicitly part of the game's design. Spend some time eliminating these annoyances and the scenario will give a much better first impression.
All tech trees and unit files begin with an initial blueprint, but the best ones are constantly tested and refined before being signed off. You don't have to get everything done in one sitting. Scenarios can take a long time to finish so you have plenty of time to re-evaluate and make changes. Little tweaks can end up having a major impact on gameplay.
Testing
Play-testing is vital to ensure your scenario has no bugs and works as intended. It is rare for everything to work perfectly first time. Be prepared to start your scenario, make an extensive list of required fixes, try again, write up another list and so on. It can be disheartening, but if you're organised and persistent most issues can be ironed out.
Once you have corrected any bugs and got the gameplay balance right you can look at recruiting a playtester or two to give you their take on the scenario. Be prepared to take on board constructive criticism. Having somebody else's input can be extremely valuable. They may pick out issues you had never considered or offer ideas for improvements that will enhance the scenario.