Put the door together and square it up by measuring from corner to corner and then the opposite corners. They should be the exact same measurement. If not “tweak” to door into square.
Using the same forstner bit that you drilled the peg holes with, mark the center of each hole on the tenon.
The step-by-step instructions provided by Runner Duck Resources here…
Great breakdown of all the steps to build your own screen door. Thanks for sharing!