What is the best way of learning APA writing if you have never written in that format?
I recommend with starting with a APA template for your writing program -- like MS Word or Google Docs. There seem to be quite a few to chose from on university websites.
Then, don't focus on writing your first drafts to conform to the APA style guides. Worrying about syntax and style sap the creative and intellectual process.
Write the best papers you can, then edit the papers to conform to the APA style guides.
If you've other papers you've written in your undergraduate, you can edit them to use the APA style. As a way to learn. There seem to be lots and lots of guidelines and tutorial on mastering that style -- many are listed in answers to this near duplicate question on Writing@SE How can a practical learner get up to speed on APA quickly?