If you are new to Lua programming, new to WoW addon programming, or even just new to oUF layout programming, I would
not recommend oUF_Simple as a good starting point. No offense to zork, but it's massively complicated. I've been writing addons for 6+ years, and maintain some very popular ones (including Grid), but it would take me
hours of reading through oUF_Simple just to get to a point where I felt like I had an
okay grasp on what was going on throughout the huge, convoluted, split-up codebase.
If you want to write your own layout from scratch, start with oUF_Classic or oUF_Lily. Both are very simple, have everything in one file, use only the core oUF elements, and do not use any third-party libraries, plugins, or modules. It's easy to see how each part works, and how all the parts work together. I'd recommend avoiding complex things like rune and totem displays until you have a solid grasp on the basic things like health bars and text.
If you're already using a layout and just want to change a few things, start there. Identify something you want to change -- preferrably something small -- and then try to change it. If you're having trouble finding which part of the code you need to change, or which functions to use, check out some of
these resources, and then post your specific question here and ask for help.