Making basic cuts, mitres, and chopping out joints.
It's aimed at complete novices, so if you already have any knowledge of woodwork some or all of this will likely come across as patronising. Sorry about that.
Sawing a piece of wood is a really easy job, but without a little training most people screw it up. Going straight through quickly is a good way to end up with non-square joints, and when you're making furniture they're a very good way to make it look terrible.
My first ever job was as a joiner's apprentice, and because I didn't enjoy it I sucked. 11 years later I find myself making basic furniture. Remembering all the advice I was given, taking my time, and doing something I want to with it, I find I don't suck so badly: Doing passable joinery only takes a little knowledge coupled with care and attention.
One important technique that doesn't need much showing: Pilot holes
When you drive a screw or hammer a nail into a small piece of wood or too close to the edge of one, it will split. Sometimes putting a fixing into a place like this is unavoidable. Drilling a hole through the joint a little bit smaller than your fixing will prevent it from splitting.
Step 1: Tools
I'll be keeping this to very basic, cheap tools:
Hardpoint tenon saw
You can be more precise when marking wood with a pencil than a pen, and even heavy pencil marks are easier to sand off than greasy biro. Use light pencil marks ;)
A hardpoint tenon saw, unlike a proper one, has to be thrown away once it becomes blunt because it can't be sharpened. However, if you only seldom make joints, it's all you'll need and very cheap. I think the expense and maintenance of a good tenon saw is only worthwhile if you're going to actually use it a lot.
The square you see in some of these photos is an old, cast iron Rabone one. I don't think you can get them like that any more, but there's plenty similar available.
Pump cramps are one of the best tools I've found: Inexpensive, very quick to apply and remove, and two are secure enough to hold most work pieces in place. Put cardboard between the cramps and your workpiece, because sanding or planing the marks out later is really annoying (see pics 3 and 4).
As you can see below, I've improvised a workbench from an old chest of drawers. Be careful not to hurt your back if you do this (or use a workmate type thing), a proper workbench is a good deal higher for this reason.