I'll start by saying that, I love to learn chess theories and do my opening preparations. In a tournament, I usually play 1. d4, and expecct replies like, Slav, Semi-Slav, King's Indian, Nimzo, Tarrasch defence. So I have not studied in depth lines like the Catalyn, Benoni, QGA, Queen's Indian yet. Trying to play one opening and be good at it, then can shift to maybe 1.e4 again.
So, this once I faced a rather unsual reply to 1. d4 which was 1... c5 and I was successful to 'Transpose' it to the tarrasch defence pawn structure which I am comfortable with.
Another game I played with another higer rated opponent,
The key idea how to transpose is to just have a look at possible openings, and understand what kind of structure is good. Know various openings or at least their structures and when you study the opening you want to perfect in, you'll definitely come across "How to get to this position ?" from another opening.
Say for example, how to get to slav from english ?