I've always found SK ratchets one of the easier ones to dissasemble and reassemble.
First, clean any gunk out of the groove in the mechanism with a pick. Do this by putting the pick in the groove and spinning the mechanism. Gunk in the groove can prevent the spring from compressing.
Use a small pair of bent-nose pliers to pinch the circlip and pull out the mechanism. I have a really cheap pair I bought new for only a few bucks.
To reinstall:
Put the selector lever in the neutral center position.
Put the ratchet handle in a vise if you're having trouble holding it.
I put my finger through the hole in the back of the ratchet head to hold the selector cap in place, since nothing attaches it to the mechanism. Then pinch the clip and the mechanism should fall back into place.
Easy as pie.