Not sure how to successfully drill these pieces
As the diagram mentions "pocket screws" you typically drill those type of holes using a pocket hole jig.   Here is one of the more popular ones, available from various outlets:  Kreg Pocket Hole jig  https://www.homedepot.com/p/Kreg-Jig-Pocket-Hole-System-R3/202269070

You can clamp the jig to the material and it holds the drill bits at the proper angles to easily drill the holes where you want them.