I'm a newbie when it comes to R/C vehicles, but an old hand when it comes to DIY electronics (20+ years)
I have 3 soldering irons, and just last week I used my butane soldering iron to put on
XT90 connectors to my new batteries.
The best tips I can provide are:
Prep is the key- Get everything ready before you start.
0. Eye protection- safety first
00. Electrical tape- block off the terminal you're not using, to avoid accidentally touching the + & - terminals of the LiPo battery and causing a short circuit
1. Make sure iron full butane (don't run out halfway through a job)
2. Something to clamp the XT 90 connector still (eg. Eg. Soldering ''Helping hands', Heavy wrench, or pliers with rubber band)
3. Don't spend too much time on the cable whose other end is connected to the LiPo. After turning on the iron, turn it down, and use just enough heat to melt solder.
Watch a few videos on YouTube to get a feel for difference approaches.
Here's one way of doing things (despite being in French)