How do you do a monthly budget?

Luckily, I don't need to now but budgeting helped me through long, difficult years. By planning what I HAD to pay and taking it off my earnings, I knew how much was left for optional things ... like heating or decent food.  We all have wants ... far more than we can afford .... so we have to ask ourselves "What makes THIS thing my top priority at this precise moment?" We have to accept "Temptation is not justification". You learn to go without a lot but you also learn resilience. A budget is there to guide you but it is YOUR strength that makes it work.

Money in should be a bit more than money out. Keep records.

You have to have the self discipline to stick to it. 

I note what I can expect to be the same or similar every month - Rent, health insurance,
and electricity. I allow myself a certain amount for food every month, though am careful
not to exceed it. Those are my main expenses - don't have a car, or loans to pay back. 
When selling things, it goes directly to my savings account and pretend I don't have it
as to become emergency funds - Very important as I've been unemployed since March.
EDIT - Excellent point by John .. by keeping track of expenses, one has a better idea of 
how to budget & what they HAD to spend whereas what might not have been necessary.

I failed for years.. Eventually I figured out the secret. You needed to have more income than expenses. As long as I was living hand to mouth, or going in debt, a budget didn't work. Poor working folks can't budget. We are too busy surviving. It's easy to give financial advice when you have money.

Google Dave Ramsey.  He also has a budget app called “every dollar”.

I stopped smoking pot and got a job