How do I set up paypal on form to accept donations?

How do I set up paypal on form to accept donations? And can 3 or 6 instalments for 1 payment be accepted?

When you say that you want to accept the payment in instalment, it seems you want to receive recurring payments. 

Yes it is possible to achieve your requirement. You may like to take a look at the following guide which should help you:  https://www.jotform.com/help/166-How-to-Setup-a-Recurring-Payment-Option

When you setup recurring payment option, it will allow you to setup number of payments as well.

Hope this helps.

Do get back to us if you have any questions.