Talk to me like I am 5. How in the heck do I account for improvements for rental property right before selling it?

I keep seeing folks say, just add them to your cost basis - but TT explicitly states one should NEVER adjust the original cost basis (it's already populated as it transfers from previous years). 

So, exactly what does it mean when I read other posts that say to do that?

All TT gives me is a place allocate sales price, sales expenses (which wouldn't include, say, a new roof), land sales price and land sales expenses.

So how would I account for the cost of the new roof?

I thought about adding it as a new asset, but that makes no sense since it's being sold just weeks later.