Scratchbuilding 1:700 scale vehicles.

How would I go about resin casting? I've never worked with resin before

I have scratchbuilt some in 1/700, using styrene strip and stock shapes, but only one-offs, as per the pics below. If you are looking at making huge or even moderate-sized "battle" dioramas, and don't want to spend the rest of your natural life making them, I think that resin casting is going to be the way to go. Make a good master and then pop out as many as you need.

Of course, there are ample vehicles available, for some of the countries, in the desired scale, but that gets real expensive, real quick. Below are two of my efforts, a daihatsu landing barge, and some railroad rolling stock:

The simplest materials for your project would be stripwood.  The width and height of the vehicles would be pretty small.  Get the next largest size of stripwood and sand to correct width and height.  Carve vehicle on end of strip, cut off, and start next one.  The model railroad section of the hobby shop will have stripwood in lots of sizes.