I guess you have to practice... I must admit I just played around until I understood how to do stuff.
Basically, you have to use the "Pen" tool. You can adjust the smoothness to suit your needs.
Then edit your lines with the "Node" tool (second one on default layout).
I almost don't use the rest actually, and just google it when I need something precise
This tutorial seems to be rather okay (and it seems the guy really goes into details)
Otherwise, you can go on the
Bscotch Youtube Channel watch and learn by looking at
Sam's Timelapses 
Timelapses can be good to see how people use the software, and you may find some from other people too to see different methods
