When you sign your collaboration agreement or memorandum of understanding with the client, it should include a payment schedule.
Typically the payment schedule is a percentage upfront for expenses; a payment halfway through, and a hold back for the produced video.
Percentages could be one-third upfront, one third halfway through, and one third holdback.
Prepare a form for an invoice which includes and identifying number; date; details of what's being invoiced, and payment instructions.
If the client doesn't pay within a month, mail or email another copy.
Sample Invoice