Don't turn the iron up too hot.
Tin the wires first.
Flux everything. If you need to rework, add more flux. Flux boils/burns off fairly quickly, and the residue left behind is not flux.
Clean your work so you can inspect it.
If your wire insulation has a low melting temperature, 1: be quick about it, and 2: strip the wire shorter than you need, because It will creep back as you're tinning it.
Google for "NASA Soldering Spec" for instructions and pictures of what "perfect" looks like. All of the NASA specs are free public info.