Soldering is pretty easy, but soldering *well* can be hard!
As has been mentioned, if you want to get good solder joints, you need to quickly heat the wire/lug to a temperature which will cause the solder to liquefy and flow into the joint, something on the order of 400F. The soldering iron tip is much hotter than that, so the solder will liquefy quickly when it touches the tip, but if the pieces being connected are not heated to a temp where the solder will be liquid, it won't flow into the joint and make a solid connection. That's when you get lumpy connections. If the things being solder are way too cold, then the liquid solder will crystallize when it hits the joint, which makes a dull-colored cold solder joint. These are really bad because they have higher resistance than a correct solder joint. That won't matter much at the low voltage of your guitar's electronics, but if you make a joint like that in the hot connections of a solid state power amplifier, the extra resistance will cause those joints to heat up.
You have to make the connection hot, then let solder flow in, and hold it completely still until the solder has cooled, otherwise you'll again get a cold solder joint.
Solder used to be 40% lead, but lead is frowned on these days because of toxicity, so the solder you buy today is a usually a different mixture which requires more heat and is a little harder to solder with. You can get a better connection with silver solder, which requires even higher temps to work with.
Guitar parts are somewhat harder to solder than a PC board. Getting a connection on the back of a pot means heating up a very big piece of metal compared to a tiny PC trace on a board. Soldering 1/4" connectors are particularly hard because there's a lot of metal again (and there will be physical stress on the ends later). Fortunately, the 5-pin XLR connector used by Series basses are pretty easy as you're only heating that little solder pot connection.
Since this is the Alembic board, I shouldn't get called down for saying that the equipment that you use makes a huge difference. You can solder guitar electronics with a 35w solder pen for $20, but it's hard to quickly build up the heat you need. A temperature controller soldering iron costs a lot more, but can put a lot of heat into a joint to make a good connection and uses special tips so you can dial in the temp you want. I use a really old Weller WTC which was an old standard for electronics.
Your "hygiene" techniques make a huge difference too. Electronic solder has a rosin core. When you heat the solder, the rosin liquefies and flows over the joint which cleans off oxidation in preparation for the solder (this is what's making the smoke too). You can buy soldering flux which is like a paste, wipe it on something like a 1/4" connector and it will really help make a clean and effective join. Also, the hot tip of the soldering iron gets covered with oxides as well, which don't conduct heat very well. So you really want to clean the tip of external oxides, then touch a tiny bit of solder to coat the tip right before you make the connection. This is the purpose of a sponge or the little ball of golden tinsel that you use to clean the soldering iron tip before use. Easy and inexpensive to use and makes a big difference.
