The project needs to start at step 1. I downloaded the CAD model as an Inventor 2019 assembly. I'm using Inventor 2020, so the parts have been migrated to 2020.

Looking at the assembly, it's a bit disorganized, with respect to the subassemblies. I will have to rearrange the structure of the subassemblies. Having a clean and organized assembly and subassemblies will help with making changes. 

This is the model tree as downloaded.

Here is the structure of my model. Inside these subassemblies are more nested subassemblies, that I will further clean up as I go along. Whenever I do design, I always attack the area I consider to be the simplest, it helps gain project momentum at the start and allows me time to do some incubation engineering the more difficult parts. Plus, later down the line, if I get to a point where prior design decisions have to be changed, I'm likely only changing the simple parts, not reworking the complex parts.