What you do in the video is just ... acceptable.
One observation ... you don't clean the tip on the sponge. Add a bit of solder on the tip if there's not much, get a paper towel or "chemwipe" (lint free handkerchief sort of) , clean the tip with this paper towel/ whatever (run the tip over the paper and rotate it a bit). Now that it's clean of solder, rub it a bit on the wet sponge to break whatever oxides are still there, and put a tiny amount of solder to minimize oxidation. You're ready to solder. When you're done, add some solder to the tip to protect it until next use (you'll remove this solder with your paper towels next time)
The technique is not that great because you have little control over how deep the solder actually goes in.
Pace Worldwide (they make soldering stations and lots of soldering related tools) has on their channel very good videos. Among them there's a series called "Basic Soldering Lessons" which may seem dated, but the information in the videos is still quite valid. You should definitely watch parts 1, 6 and 7 as a refresher, and below you can see Lesson 3 which is about cup terminals (similar to what you have on those D connectors)
The basics : TIN the wires first... it's important. Fill the cups with solder (bend solder wire in 2 or 3 or as many times as needed to get thick enough solder to get right amount in cups). Heat until flux boils at the top then insert tinned wire, remove tip and hold in place until everything solidifies.
This guy does it the paranoid nasa avionics grade ultra ridiculous whatever way - seriously you'd probably roll your eyes half way into the video :