If I'm giving them a bath I clean between the teets with water. Otherwise I just use my fingers to kinda peel/rub out the gunk. Be careful when you do it. Once in awhile I get a squeel and one will attempt to kick. All my girls like it once they figure out what I'm doing, I think it must feel good. I do it about once a week.
I have to add, DON"T under any circumstances smell your fingers after you do this. Its awful. Yes, I know you will