Let's say you are starting to build a form with many trivial elements and simple back-end logic, but also with some complicated blocks that you know will require lots of trial & errors.
What do you start with, the easy or difficult ones, and why ?
Unless I have already a clear path (generally a wrong assumption) on the more complicated parts, I start with the easy ones so to have a little positive reinforcement when I can see they work !
It depends on who else is involved. If someone else is going to need to hook up the form in Rails or something, then we'd get those parts done first. If we needed some information on the complex bits - and had some time - we'd do the layout / general stuff.
We usually make a CodePen for anything difficult - so that it's all by itself and we can save it. So - if we had to choose / we'd do the harder parts first - and get them out of the way - or get to a point where we decide to hire that out - or reach out to the library developer etc - and get a feel for any gotchas we might run into. Hard first!