Choose Columns in Datamart

I'm using Datamart to write queries and I'm new. I have seven tables merged in different ways. I need to choose columns but want to do it the most efficient way possible in case I mess it up and have to do rework.

Should I choose columns in the original tables or in the merged tables? Which is most efficient and easiest to track back to?

Also, I need to merge two columns, but I want to keep the original two columns, a total of three columns. I know how to merge, but the original two columns were then gone. How do I keep all three?

Thanks!