The idea is most important, dont try to draw something if you don't know how it looks. Also the mapping - you need to think and imagine how you'd like your bicep for instance to look like, what will be in front, what in the top and draw it (use the templates so you know where to put elements)(there are sets with semi-random elements and they look nice but in my opinion you need to be experienced to create a set/texture by improvisation).
Don't try to shade and finish the texture first, you need to draw the shapes, eyes, visor, maybe a hair or something and check out how it looks, are the eyes too separated or not, etc. Do a sketch first and when you know you've drawn all the elements or the most of them, shade and finish it.
To preview a head you may use a 3d program like 3ds max, it makes easier to preview and test the head and you can check out how it looks in 512x512.
But of course if you don't know 3d programs you can always upload the heads to toribash, it's a good way too.
You may use some reference pictures, a hero or monster so you have an inspiration.
Test the texture as often as you can and imagine, think what can you add.
Check some tutorials on shading to make your textures look better
If you don't know any drawing program but ms paint, download program like gimp and do as much tutorials as you can, an abstract background tutorial, text effects tutorial, drawing tutorial. By doing this you learn the basic structure of the program and you know how to use tools, filters, blending modes.
good luck and don't give up