Here's how I like to get the buttonhole placement on a shirt just right, tips on sewing buttonholes and a tutorial link to how to sew on a button.
How to Sew a Bound Buttonhole
4 Positioning Points for Perfect Button Placement – The Daily Sew
Sewing A House (or Tower) Tailored Shirt Placket
Shirtmaking: Button Placement & Buttonhole Tips
Shirtmaking: Button Placement & Buttonhole Tips
Diary of a Sewing Fanatic: Shirt Sewing Tips
The Perfect Blouse: Make the Buttonholes - Threads
Diary of a Sewing Fanatic: Shirt Sewing Tips
Classic Button Up Shirt Sew-Along - Part 1 - WeAllSew
Shirtmaking: Buttoning Down the Basics EmmaOneSock Sewing Tutorials
Sew Alongs and Tutorials