There are many tools you can utilize to develop Power Automate custom connectors including Postman and Swagger Inspector. I prefer to utilize Postman but recent updates to the product no longer make it possible to export to a v1 collection.

This is unfortunate because the Power Automate custom connector site only allows upload of v1 collections.

Power Automate custom connector import page showing the Postman collection v1 upload requirement

Luckily APIMATIC allows you to convert the Postman 2.0 collection to just about any other format you want. You can downgrade them to Postman 1.0 or you could convert them to OpenAPI 2.0 format which you can also directly upload to the Power Automate connector page.

APIMATIC format conversion page converting a Postman 2.0 collection to a compatible connector format

Have fun creating custom connectors!