Learning how to wrap your head around a construction project and come up with an accurate budget is a critical skill in successfully pulling off forced appreciation.
The first step is to get your bearings on the project. You will want to take photos, maybe a walkthrough video, and get measurements.
I found that the best way to do this is to work through the project linearly through time. Then, working through the construction process, write a breakdown of the basics to do in each step.
There are items that you can scope in multiple ways. For most projects, this will likely be too much detail, though. As you work through a few budgets, you will start to find a middle ground of detail.
You will need to estimate both labor and material costs for this part of the project. This process will take a tremendous amount of time the first few times you go through it.
I mostly work in a single spreadsheet, and then on complicated line items, I link to a secondary supporting worksheet with further breakdowns and descriptions of the work.