This is a downloadable, free program, as easy to use as the MS Paint, but also with more helpful tools.
Basic Micro Making in Paint.NET
Open the template, use the Wand tool with the background, press the "Delete" key to obviously, delete it. Open the reference micro or image, because you can open several images at the same time, and whatever you may need to do the micro.
Do the micro normally, as in MS Paint. Tools as Color Replacement and the Wand should help.
Remember, when you want to copy one half of the micro, turn it, and paste it in the other, make a new proyect, because everything turns.
Also take a look to the Effects, they are very helpful.
Use the common Mask tool and cut the micro as good as you can, and paste it in a new proyect, don't worry, the new proyects are as big as the images you cut or copy.
Congratulations, your micro is ready!