Driving screws or nails without wandering
The first thing I ever screwed together came out with every screw pointing a slightly different direction, like each one had its own idea about where to go. By now your glue's dry, or close to it, depending on where you landed in the schedule, and today we put metal into this shelf. This is the lesson where I see the most wandering screws in a beginner class. Not because it's hard. It's because people rush the start of the hole, and the screw picks its own direction from there.
A screw or a nail follows whatever path it starts on. Start it crooked and it stays crooked, and there's no straightening it once it's three-quarters of the way in. So most of what I'll tell you today is about the first quarter inch. That's the whole ballgame.
Starting the hole square
You should already have your pilot holes drilled from a couple lessons back. If you skipped that, go back and do it, because driving a screw straight into bare pine with no pilot hole is asking for a split, or a screw that wanders because it's forcing its own path through the grain instead of following a hole you already made for it.
With the pilot hole there, set your screw tip right into it. Before you pull the trigger, look at the screw from the side, then look at it from the front. Two angles, not one. You're checking that the screw stands up straight to the surface of the wood, not leaning toward you or away from you or off to one side.
This is where the square earns its keep again. I've said it before in this class and I'll say it here too. Mary bought me a decent square for my birthday years ago and it's the tool that changed the most for me. Before that I was eyeballing every right angle and wondering why nothing ever quite lined up. You can pull the same trick here. Stand your square up next to the screw, not touching it, just as a reference line for your eye. If the screw looks parallel to that square edge, you're square to the wood. If it's leaning, you'll catch it right away, and that's the moment to fix it, not after you've driven it an inch deep.
Driving it
Start slow. Slower than feels necessary. The first turn or two sets the direction, so don't gun the drill right out of the gate. Get the tip biting into the wood, keep your eye on the angle, and only pick up speed once you can feel it tracking straight.
Keep steady pressure straight down the line of the screw, not off to one side. If you're using a drill, let the tool do the work. You're mostly there to hold the angle, not to muscle it in. Using a hammer on a nail, it's the same idea. Light taps to start, make sure it's standing straight, then longer strokes once it's seated in the wood and won't tip.
If you feel the screw start to lean partway through, stop. Don't try to correct it by shoving the drill the other way while it's still spinning in the hole. Back it out most of the way, check your pilot hole, and start again slower. It's a little annoying, but it beats fighting a screw that's already gone sideways.
A note on nails, since some of you are doing a nailed back panel
Nails don't have threads to grab the wood the way a screw does, so they're a little more forgiving on angle and a little less forgiving on force. If you're a hard hitter, ease off. I've seen more bent nails come from someone trying to finish in three strikes instead of eight.
One thing I want to be plain about. Keep your other hand well clear of where the hammer or the drill is swinging or spinning. Sounds obvious, but I've watched people brace a board with a hand right next to where the bit's about to break through the far side. Give yourself room. Nobody needs a trip to urgent care over a shelf.
If it's already wandered
If you're partway in and you can see it's crooked, and it's not too deep yet, back it all the way out. Don't try to steer a screw that's already committed to a bad angle, it just chews up the hole worse. Fill that pilot hole path with a little wood glue and a sliver of scrap, let it set for a bit, then redrill straight through it. It's a small delay and it saves you a screw hole that never quite closes right.
People get embarrassed about this step, like it means they did something wrong. It doesn't. It means you caught it, which is the whole reason we go slow in the first place.
Before next time, look over your screws once they're all driven and see if any of the heads are sitting proud instead of flush. We'll fix that first thing next lesson.