integrate PayPal in the form

Could you advise what steps to take to integrate PayPal and other related advices.

You want to integrate with PayPal (Standard), here is the detailed guide explaining the integration process: https://www.jotform.com/help/391-How-to-Integrate-Form-with-Paypal-Standard-

You can also integrate your form with PayPal Pro, PayPal Checkout and PayPal Express. You can see all of the integrations in the Payment section of the Form Builder.

Here is the guide: https://www.jotform.com/help/50-PayPal-Forms

Let us know if you face any difficulties.