I'm not a pro, and I've disassembled a freewheel (it had sand in it), cleaned it, and reassembled. I used a sticky grease to hold the balls, and didn't find the pawls that hard to get back in place - I didn't need to use any of the old tricks with string, etc. It isn't necessary to get every last ball back in the assembly - you might lose one or two along the way, and the freewheel will work fine.