Thanks, cnield. That's a good tutorial. I use Paint.net (which is totally different than Paint) because I find it easier to use than GIMP. Either should be fine for doing basic uniforms. Paint.net is free as well. I only started making jerseys a few weeks ago as well.
And before I forget, thanks to Deft and Ambermonk for the templates. They make it easy to create good looking basic uniforms.
I basically just paste in the flags but for some I've had to center the flag and do some filling around the edges or separately paste in elements from the flag (see the DROC below - the blue, red, and yellow one). Some flags just won't fit well into a basically square jersey area. But for most I just resized and pasted. The jerseys that have two or more colors at the top take a bit more work as well so the colors wrap around correctly but most jerseys (flags) only have one color at the top. I have several shading layers I use randomly (to put the wrinkles in the uniforms and to make them look more realistic) and I add a little bit of noise so that the colors are not so uniform.
For the caps, I took the country coat of arms, resized it to 150x150, and picked one or two colors from the flag as cnield described. Unless you want a tricolor cap or something else nonstandard, that works well.
Here are some of the African teams which I did this morning. I got the flags and coats of arms from Wikipedia -
List of African countries by population - Wikipedia, the free encyclopedia . I've done the top 16 countries so I didn't do Angola.