Hey guys.

Amateur electronics enthusiast here.
Been making a few mods to my guitars but they all seem to fall flat at the same point: Soldering to the back of pots.

I was making a stereo/mono box and couldn't connect my grounds at the switch's casing (just like a pot).

Does anybody have any tips for doing this or anything that may be beneficial to other beginners?

Get a good iron that can put out some heat for the back of pots. I would recommend a Metcal smart heat kind of setup they do a much better job. (they do cost money but I got one from work) If you have a Iron that has a few different tips use the beefy one for the back of pots and the smaller one for connecting leads. Make sure the solder flows and is shiny dull solder is sign of a cold joint that will come apart later.
I use an adjustable 40w iron with an iron tip; it goes up to about 45w, and it works OK provided the surface is clean, I also have a bigger iron for heavier jobs. I sometime use zinc chloride soldering fluid, but only where I can do a good cleanup job afterwards, due to its corrosive fumes. It is also good for cleaning soldering tips.
The bigger pistol shaped irons are better for that kind of thing. Mine is a Weller probably as old as I am, at least 50 anyway, that I bought at an estate sale. The smaller pencil type irons don't put out the power you need to solder to a box body or back of a pot. I'd have to go dig it out, I think it's 100/140 watt. It's stored in a shed, rarely need it.

Also, try hitting the surface with some sandpaper before soldering, that helps especially with chromed finishes. I use a foam fingernail file a lot, works great. If you use sandpaper, 180 grit will work fine, all it takes is enough to rough up the surface and get rid of the exceptionally smooth surface of chrome. With an effects box, it might also have impurities, use a little alcohol or acetone to clean it first. Acetone is used in fingernail polish remover, check the label for one that says 100% acetone. I get mine at the local dollar store. Don't use the ones that don't say 100% acetone, they also have chemicals for scent and color.

I also use flux instead of flux core solder, it works better, for me at least. Rosin never acid flux. Acid flux is for things like copper pipe and will make electronics rust like crazy. Flux cleans the surface and helps solder flow better. Let the iron heat, and clean it after every soldering with a wet paper towel or natural sponge. A clean iron works a lot better than a dirty one. Also melt a tiny blob of solder onto the tip before soldering, it helps transfer heat better.
Hmmm...I wonder what this button does...
If all you have is a plain, cheap iron from walmart or wherever, clean the back of the pot with #0000 steel wool, a file, or scrape at it with a knife. That'll give you a clean spot to solder to. Apply a dab of solder to the tip of the iron to draw the heat, than just hold it to the pot casing until you can start feeding solder. Once you have a good solder blob on the pot, let off and start tinning your wires. Once all the wires are ready, place them on top of the solder blob and then put the iron on top of that, melting the solder on the wires and the pot in the same operation. You'll want to hold the wires with needle-nose pliers for this.

It's tricky (and a pain in the arse) to do with the cheap unregulated irons and you pretty much only get one shot to get all your wires onto the pot casing since each new wire will carry away the heat from the iron. Last year I finally decided to splurge on a Weller adjustable pencil with an iron tip and I'm never going back. Just clean the pot, dial up the temp, and go to town.