@ ultimate - Raliuven´s plan will work, but it´s not optimal.
the main point is,
irrigate the FP wheat!
plus mine 3 bg.
build a granary.
that´s it for the SF from size 4-6.
turn 1 (town size 4, granary half-filled, no shields in box):
city center, 3 mined bonus grass, irrigated FP wheat: +5 food +7 shields
turn 2 (town size 4, granary 3/4 filled, 7 shields in box):
city center, 3 mined bonus grass, irrigated FP wheat: +5 food +7 shields +1 shield on growth
turn 3 (town size 5, granary half-filled, 15 shields in box):
city center, 3 mined bonus grass, irrigated FP wheat, e.g. river grass: +5 food +7 shields
turn 4 (town size 5, granary 3/4 filled, 22 shields in box):
city center, 3 mined bonus grass, irrigated FP wheat, e.g. river grass: +5 food +7 shields +1 shield on growth
this sums up for 20 food and 30 shields every 4 turns exactly. you lose 2 pop points on growth to size 6, thus falling back to size 4, and the town looks exactly like described in turn 1 again. if not, you made a mistake.
[in case i just err on this, not having the game in front of me, and an irrigated FP wheat gives only 4 food, you´ll need to go size 5-7 with your SF including working the grassland wheat all the time for 1 additional food. or you irrigate it and share it with another town and work it only every 2nd turn.]
of course
roading all tiles you use will help, but not your SF but your commerce production. therefore, make sure to always work (roaded) river tiles over (roaded) non-river tiles, too.
just try it out, and look at the city screen every turn. handling of a SF has to be learned, and inevitably you´ll make mistakes in the beginning. this is one of the easier ones to start with, so it should go well (as long as you don´t suffer disease from the FP). good luck!
templar_x