Here is how I do it :
First I will look for a particular unit that has the parts that I want. Using the canoe as an example, I took the coracle graphic as posted by Kal-El and edit the coracle by removing the cover for the coracle, shorten the boat and recolor the part where the cover has been removed to get a base for the canoe.
Next I need a person who paddle the canoe along. I need someone that looks like a native Indian, the closes of them being the ancient worker. So I grab the ancient worker graphic and remove the legs, then I add new legs by drawing them in so the new graphic has the worker in semi-kneeling position. I then change the shovel into an oar and added a feather to the head.
Once this is done, I use the magic wand selection tool in PaintShopPro to select the background and then invert my selection. After that, I will just copy the inverted selection (ie. Anything that is not the background color) and paste it over to the base canoe graphic that I created from the coracle. For certain view where part of the canoe is going to cover the Indian, I will first copy that part of canoe and paste them into a separate file. This way I can repaste the selection back to the canoe frame after I paste the Indian in. As to each of the paddling action, I draw them frame by frame for the hand part only.
In fact, its just like Kryten has mentioned, cut and paste is the best tool to use when making an animation. No matter whether you are making them from scratch or copy from an existing one. Just remember that animation is a loop and the easiest way to make a loop is copy and paste. A simple loop will be as follow:
1->2->3->4->5->4->3->2->1->6->7->8->9->8->7->6
In the loop above, its 16 frame but actually only 9 unique frames. So, to reduce your work, after making frame 1, duplicate a copy then only make frame 2 between the two frame 1. After that duplicate frame 2 and make frame 3 in between the two frame 2 again. Hope this helps with all your animation needs.
