When wiring a European (Schuko) plug, what thing should I focus on not to mess up?
I assume the plug uses screw terminals. In this case, and if you use stranded wires, use wire ferrules to protect the wire strands from breaking.
The wire must be rated for 16A usually.
Ensure that the strain relief is tightened properly.
Make the earth/PE wire little longer than N/L. This is to ensure that the PE wire stays connected longer than N/L in case someone manages to pull the cable out.