1) Clean your surface. Dirt and grease can ruin a solder joint. Home improvement scotch brite pads are great at this, but do not use kitchen pads as they have soap built into them. Rub them down and spray them with compressed air.
2) Secure your work. Even slight wiggling can lead to poor solder connections which can lead to shorts or lost current. (Think creatively with vise grips and fishing forceps).
3) Before you use your iron, heat it thoroughly, then apply solder to the tip. This is called "Tinning" and it helps conduct the heat and burns off the chemical coating that comes on your iron from the store. Also a good tip is to keep a DAMP sponge handy when you solder. If you get any dirt or solder buildup on the tip of your iron wipe it on the sponge.
4) Heat the wires or joint for a few seconds before adding the solder. You want the metal to bond with the solder. But don't leave it so long you burn the wire coating. Practice on scrap wire can help get you a feel for heating time.
5) Don't add too much solder. You don't want a gunky joint.
6) It only takes a few seconds to solder a joint.
7) Do not move parts until solder has set.
Important note that many people do not know about. FLUX. Flux is a compound that prevents oxidizing. Flux is added to many electronic solders. Have you ever gotten spikes on your solder where it tries to stick to the iron? Lack of Flux can cause this. A problem many people encounter is heating the joint too long and burning off the flux that is in the solder. Flux is sold in electronic stores and craft stores with stain glass area. Adding a little of this Vaseline like stuff (usually with an included brush) can quickly give you an easy to work with, smooth surface. Thus, making your work look clean and neat. Some solders do not come with flux embedded and can cause headaches for the worker from the get go. For electronic work look for solder with a "Rosin Core." This is Flux. Buy separate flux also in case you do burn the embedded stuff off. It will save you many headaches. I prefer to work with non-cored solder. But I have done this for a while. When you are done be sure to use rubbing alcohol, wood alcohol or antifreeze to clean off any white residue. This is left over flux. If left on wires it can absorb moisture (and many types contain low amounts of acid) and will eventually erode your wires. Solder has lots of chemicals in it so be sure to work in good ventilation.
Shrink tubing is also a bit tricky in that there are different types. There are adhesive lined and non-adhesive lined. Non-adhesive forms a barrier only and is good for wires with no exposure to moisture, chemicals or dirt. Adhesive lined has heat sensitive adhesive that melts to seal the joint and should be used for areas exposed to the elements... ie... most anything in a car. Inside or out. Use shrink tubes 1.5-2 times bigger than the wires. Shrink tubing should hang over the joint by about half an inch. Use a heat gun or hair dryer to shrink it. In a pinch a lighter will work but isn't recommended.
Solder won't "stick" = dirty surface, use iron and a desolder wick (something else to buy) to clear the solder and clean the surface. The surface might not bond to solder... aluminum.
Gray grainy bond = either the joint was not heated enough or the joint was moved before completely cool. Some "cheap" irons may not get hot enough for big wire jobs. Lots of metal to bond creates a makeshift heat sink that is harder to heat.
You get "Spikes" = overheated and burned off your flux. Add more Flux.
All you pros out there feel free to add bits of learned wisdom. The best way to learn to solder is to get a vise, some scrap wire and start experimenting. Also Youtube probably has a ton of how-to clips. Have fun and sorry if nobody gives a shit about this.
P.S. Most of my knowledge comes for years of doing
stained glass. If you can find a local class its actually
a great skill set. You learn to cut glass (yes there
are tricks to that too) and solder... plus you can make
lots of badass cheap Christmas presents.