When you don't know what you're doing it is always a struggle.
Likely you didn't have to take the whole thing apart to thread a new cable but again when you don't know what you are doing it is always much harder.
Generally all you have to do, as has been said, is to release the spring tension of the shifter so the cable port is visible...sometimes you have to remove a top cap or a bung screw/plug to reveal it depending on brand/model...then just insert the new cable.
The soldering thing is a total waste of time and just complicates the whole thing.
Good luck...you are going to need lots of it...