Did you do nap training and how did you go about doing it?

DD is 6 months and is still being rocked to sleep. We did some nighttime sleep training and she will put herself to sleep but we haven't approached naps. Recently though her naps have been getting shorter and shorter to the point where she takes 4 naps that are 20 minutes long. She wakes up crying, tired, and rubbing her eyes (poor baby ) I wouldn't have a problem if she seemed ok with them but she is so tired and I feel bad. I feed her after her naps and she frequently sleepy feeds for a long period of time. Any suggestions?