There is a program that will turn a equirectangular map into any projection. Therefore, you should make the map in equirectangular and if you need to reproject it, you can use G.Projector. It's quite useful and easy to use, but your map needs to start in equirectangular.

In case you have no idea what an equirectnagular projection is, here is the wikipedia article. The basics of it though, is that x=latitude and y= longitude.