Q: use rejection sampling for true random number generation in a range (entropy from radioactive decay)

Hi everyone I have been doing some reading and have come across true random number generation using the entropy from radioactive decay. I have written a helper tool that returns the next random byte. It uses a server that provides bits from such a setup, I believe its data is from cesium decaying. I have done quite a bit of searching and have not really been able to figure out how to go about using this to generate numbers in a range from 0..n-1.

A user on the unofficial SO irc told me this

if you have a random byte, 0..255 evenly distributed and you want a random number in the range 0..5 there are 6 values in the output range and 256 in the input range the greatest multiple of 6 that is <= 256 is 252 so you would sample your random byte until you get a number in the range 0..251 then you could take the number MOD 6 to get your output number.

Im not really sure how to sample the byte. Do I use a single byte or do I have to continually request more bytes? Im really just having a hard time rapping my head around this, so any thorough explanation not using obscure math notations would be extremely appreciated.

Thanks.