The basics are this: you use one paper envelope per budget category. On payday, you get cash and stuff each envelope with the money earmarked for that category. When the envelope is empty, you stop spending.
First, most of us aren’t used to using actual dollar bills and coins. Secondly, many stores prefer cards over cash. And third, most of us get paid by direct deposit, and going to the bank every payday to get some money for your budget isn’t very convenient.