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.