For me, GTD is a series of workflows and habits. I repeat my Daily Review, Weekly Review, Monthly Review, Quarterly Review, and Annual Review. I broke GTD down into a group of checklists that I can follow. GTD was very difficult in the beginning because I didn't adopt the habits yet. Having checklists helped me with that aspect. When I turn my car into the shop, the mechanic has his checklist. He checks off everything inspecting starting from headlights, tires, wiper fluid, and who knows what else goes into a car? He probably would have missed something if he didn't go through his checklist.
I took a lot of the checklists from GTD and then slowly tweaked them over time to fit my system. Everyone's GTD checklist will be different. Instead of looking up the book for the checklists for the different GTD workflows, I thought it was time to make it my own. I had a sample here.
https://community.effectiveremotework.com/t/do-you-have-your-personal-gtd-checklists-ready/5615
I tried to remember all the steps in the daily review and weekly review first. I failed because I would eventually miss something. Having a checklist provides consistent results. It's always on my mind when I'm unsure if I missed anything. The checklists are in front of me and I can focus on the contents of my task manager (projects, lists, next actions) instead of worrying about the actual workflow itself.
Checklists were my key to documenting/customizing my workflows. Every workflow is consistent and it reduced my tendencies to falling off the GTD bandwagon.