how does writing a check at Walmart work?

You do not even have to fill the check out, just hand it to the cashier, he/she will run the check through the system. If you have ever written a bad check at Wal-Mart, it will be declined. If you have no money in your bank account, it will be declined.

If it clears, the cashier will hand you a receipt to sign and a receipt for yourself.

They process immediately, guessing that will change whether you plan to do this or not.

With a pen.

why do you need to write a check?
are you sending something via wesyem union?

You fill out one of your own checks with the name of whom you are giving it to.
Fill in the dollar amount in numbers and writing. Sign it and hand it to them.
Make sure your account has enough money in it to cover the check. Otherwise,
you will be charged two penalties. One by Walmart and the other by your bank.

I wouldnt

I would imagine much the same way as everywhere else.
I must ask - what kind of backward country are you in that you still use checks (the correct spelling is cheque)