Soldering is easy. Always keep everything clean and heat the thing you want the solder to stick too, then add solder. If things are not clean or if you heat the solder with the iron/torch, nothing good happens. Depending one how things are assembled, you can twist the wires together and then solder or tin each side, then melt them together. Both ways have advantages and disadvantages. I like slide over heat shrink (ideally glue core for exterior), twist together wires, solder, slide heat shrink over joint, shrink, done.
I'm not convinced it is the best method for trailer wiring, I prefer crimps. The solder joint will never fail but the copper just outside of the very stiff connection takes a beating from vibration or bending and often fails. Crimps provide some strain relief so the stress is spread out over a longer length and therefore takes longer to break strands.