Here's a one-day build for one of the two panniers I built for my bike.
Parts used in the video include:
J-Hooks: Ben's Cycle
www.benscycle.com/jandd-pannier-j-hook-with-rubber-protector-bicycle_mounted_rack_part_jandd_22051_rk2575/p
Flat Bar (+ 6mm cap screw + hex nut): Home Depot
www.homedepot.com/p/Everbilt-1-1-2-in-x-36-in-Aluminum-Flat-Bar-with-1-8-in-Thick-801967/204273995
Remember to use the appropriate length cap screw that will fit your bag's straps. I hope it's helpful to you and if you have any additions or changes to the build let me know how it goes in the comment section. Have fun riding and carrying things by bike.