REST and SOAP API PenTesting

Hi guys
I am looking forward to test our in house API based on above two. there two different APIs… i am not experienced in testing APIs… Anyone here to recommend how to perform a pentest on these?
Thanks

Both in black arch

https://www.owasp.org/index.php/REST_Assessment_Cheat_Sheet
https://www.owasp.org/index.php/REST_Security_Cheat_Sheet