How do you go about borrowing money from your own 401K. Any caps on how much you can borrow or how you pay it back?

This is defined by the individual plan. It can vary. Not all plans allow loans. Some only allow one. Others may impose a cap (typically you can only borrow up to your vested amount. Others define the interest rate different ways.

Payback is almost always through payroll deductions. The amount will depend on how much you borrow, for how long at what rate.

You need to talk to your plan admin, they can tell you how to figure out what you can borrow on what terms and how to apply.

The maximum loan is 50% of your vested value, up to a max of $50K. Loan terms are up to the individual plan, but they must include a "reasonable" interest rate (which most plans peg at 1% or 2% over prime), and loans must be paid back within 5 years. But as ScottG says, the specifics vary from plan to plan.