The saw came from my uncle's garage sale pile, priced at two dollars with a sticky note that just said "works," sitting next to a box of mismatched hose nozzles nobody wanted either.
Your first day
You'll need a crosscut hand saw, a scrap board about a foot long, a clamp, sandpaper, safety glasses, a dust mask, a pencil and a ruler, well under fifty dollars if you buy secondhand. Put on the glasses, and the mask for the sanding. Clamp the board to a stable surface so it can't shift mid-cut, and keep your off-hand well clear of the saw's path. Mark a straight line with a pencil and straightedge.
Place the hand saw on the waste side of the line, the side you're throwing away, and start with a few slow backward strokes to create a groove. Then saw forward and back in long, even strokes with the blade at roughly 45 degrees to the board, letting the saw's own weight do most of the work. Stop just before the offcut falls free and snap it off by hand, so it can't pinch the blade.
Sand the cut edge with 120-grit paper until it's smooth to the touch.
Most beginners push down hard on the saw to force it through faster. Don't. It actually makes the cut wander off the line more, not less. Let the saw's weight and a consistent angle do the cutting instead of arm strength.
The thing most people miss
I believed good woodworking required expensive power tools to get clean results, that a hand saw was something you settled for until you could afford better. What actually changed my understanding was that a hand saw, used slowly and correctly, produced a cleaner line than my first attempt with a power saw, rented for an afternoon and rushed. I remember staring at a wall of ladders in the rental shop while I waited for it.
Once I saw that, I stopped assuming better tools would fix bad technique and started slowing down with basic tools first, since the technique itself carried over once I did eventually use power tools.
What not to buy
Don't buy a set of chisels, planes and extra clamps before this first cut. One saw, a pencil, a straightedge and sandpaper are all this exercise needs, and a bigger toolkit just gives you more ways to skip the practice.
By about the fifth practice cut, the line came out straight enough that I didn't need to sand away any wobble, just smooth the surface itself. I held a straightedge against it afterward and there was no visible gap anywhere along it, where my very first hand-saw cut had a gap wide enough to see without measuring. Basic joinery, starting with a butt joint, one board's end glued flat against the face of another, is the simplest way two pieces of wood actually get connected, and the natural next thing to try.