Only one final advise after soldering many many connectors on my hobby business of refurbishing wiring harnesses.
1. Buy solder brand KESTER made in USA. excellent flowing properties (Amazon). What a difference compared to Chinese stuff of many brands.
2. Sand pins and then, with a small Inox steel brush, make them shine before soldering. Cleanliness is vital for a good solder.
3. Keep your solder gun tip clean with appropriate cleaning web so that the exposure time on each solder operation, is minimal due to an excellent thermo transfer properties
If you have an old Bakelite connectors with the same number of pin holes that are present on the plastic connector you are repairing, the better. Bakelite can withstand higher temperature exposure than newer plastic terminals present from year 1968 onwards.
Bakelite is fragile to impacts, but to use it to keep cables in right place while soldering is a good extra hand, good when you are developing your solder skills.
Terminals pins as said before should be very clean. No grease, or contaminants in any way.
Same advise applies to the cables to be soldered, they should be clean. You should see a shinny copper surface. If not the case, sand the cable ends with grit P240, or if you are patient enough sumerge cables tips into citric acid to remove impurities. To try to solder oxidized cable ends is a waste of time and will lead to poor quality connections.
Best regards
L.peterssen