For me personally, I had a basic understanding of Bootstrap and how it works - but I tend to learn as I do. So I setup a dummy project and just started building until I felt comfortable to jump to our main project.
The Wappler documentation is ok in general, I found it’s often written in a way that expects you to know more than you do, and then in other occasions it’s great.
The forum is a wealth of information, so you can normally search and identify solutions without needing to ask them, but if you do ask, someone is normally on hand to try and help out, if not the team directly.
I’m not the best person to provide guidance as I do tend to just dive in and get my hands dirty, and I learn best that way.
However, there are heaps of resources on Bootstrap, this is a great one: https://getbootstrap.com/docs/4.5/getting-started/introduction/ - (note that Wappler uses version 4).
Then I’d look into the principals of how Wappler works, e.g. Server Connect, App Connect (https://docs.wappler.io/t/app-connect-introduction/2840)
Once you know the basic ‘principals’ of how to correctly use Bootstrap (e.g. container, row, column) and how Wappler gets data, and binds data - I think you’ll be just fine based on what you already learnt in the past.
Here is also a good post of an ex-Bubbler moving to Wappler and has some terminology transfer within it that might help: My Early Experiences Going from Bubble to Wappler + Translating Bubble to Wappler
This could also be useful to understand the SC / AC side of Wappler: Build a full SCRUD application in around 15 minutes