I've successfully permed several of my pieces. Here's my advice:

Go to a good beauty supply store (Sally Beauty here in USA). Take care selecting the right size perm rods. I recommend going with the largest ones (usually white) for body wave. The smaller the rod size, the tighter the curl will be.—Stay away from the small ones or your hair will turn into an afro! Also, be sure you get a Cold Wave perm solution that is made especially for Processed/Treated hair. (This step is extremely important—The perm solution for natural hair will fry your unit!)

Follow all directions carefully and be sure to check the curl during the processing often. (Since you're processing already processed hair, the perm will usually "take" quicker than the printed instruction time indicated.) After the perm be prepared to have to recolor the newly permed hair with Back2Natural, or some other color, because the perm solution will strip some color out of the hair. The newly permed hair definitely absorbs color faster, so be careful when applying color. Strand test a small section of the permed hair (somewhere along the back edge) to determine how much time you'll need to leave the color on before coloring the entire unit.

Something to also note is that the hair will be MUCH dryer than it was before the perm since it's a harsh process. Applying a deep conditioner and leaving it on the hair wrapped in plastic for a few hours will help this somewhat, but no matter how much or how often you condition the permed hair, it's always going to be dryer, stiffer, harder to style—and also prone to shed more than before perming.

There are some good videos on YouTube that show step-by-step how to do perms. You should watch a few before you proceed... Best of luck! (BTW, it lasts a long time—that's why it's called a PERMANENT, LOL.)