You can do it!
Apply flux and pre-tin all contact points. Use thin 63/37 solder and a well maintained soldering iron (that gets hot enough). Having "helping hands" or some other sort of clamp to keep things in place also makes a big difference. Like anything, once you do it enough times, it will become second nature.
A cheaper alternative to "helping hands": find some scrap 2x4, drill a hole that is wide enough to fit the right angle plug snug enough without moving while soldering. For straight plugs, I would drill another hole on the side of the 2x4, so both plug types are covered. That was what I did when I started making my own cables, before I moved on to clamp-on vise, then a PanaVise.
Can't find any scrap wood? How about a shoe box that is made of paper thick enough for the job?
Applying flux is not a must, IMO, if the solder wire is good 63-37 rosin core. However, I think it does make pre-tinning a bit easier with it, especially for those who are new to soldering. Flux or no flux, pre-tinning the wire is always preferred for making cables.
Heatsink tool, as suggested by jb welder, would be a good idea. In a pinch, I would use an alligator clip if I could not find my heatsink tool.
I have been busy making cables lately, having bought a bunch of straight and right angle plugs from Next Gen. Great service.