Marking a line you can actually follow
Okay. So you've got your reference edge figured out, you've got your measurement marked with the square, and now we've got to talk about the actual line you're going to cut on.
This sounds like a small thing. It is not a small thing. I feel like this is the step where a lot of the mistakes from earlier in the process either get caught or get baked in permanently, so let's slow down here.
Mark the point, not just a slash
When you measure with your tape, don't just eyeball where the number lands and drag your pencil across. Make a small V, or a single clean tick, right at the exact measurement. A fat pencil mark can be a sixteenth of an inch wide, and on a small shelf project that sixteenth of an inch matters more than you'd think. Put your V right on the line, tip pointing at the exact spot, so there's no question later about which edge of the mark is the real one.
I know that sounds fussy. It is a little fussy. But it's the difference between a cut that's off by a hair and a cut that's off by a hair in a way you can actually see once the shelf's on the wall.
Use the square to carry the line across
Once you've got your point marked, set your square against your reference edge and draw the line across the board using the square as your straightedge. Don't try to do this freehand. Nobody's hand is that steady, mine included, and probably especially mine.
Press the square firmly against the edge of the board with one hand. Draw your pencil line in one smooth stroke, not a bunch of short scratchy ones. If your line looks scratchy and uneven, that's usually the square shifting a little as you draw. Reset it and try again. This is not a big deal, it happens to everyone, it just means slow down and do it once more.
Mark your waste side
Here's a thing I wish someone had told me early on. After you draw your line, take your pencil and put a little scribble or an X on the side of the line that's the cutoff piece, the waste. It sounds obvious once you know it, but when you're standing at the bench with a saw in your hand and you're a little nervous about the cut, it is shockingly easy to line up on the wrong side of your own mark and take an eighth of an inch off the wrong piece. I've done it. More than once, if I'm being honest.
The scribble takes two seconds and it removes all doubt.
Cut to the edge of the line, not through the middle
When you actually get the saw going, and we'll cover the saw itself in the next lesson, your goal is to cut right at the edge of the pencil line, on the waste side. Not through the center of the line. If you split your line down the middle every time, you're actually cutting a hair long every time, and that adds up across a project with several pieces that all need to match.
I feel like this is one of those things nobody tells you outright, and then you spend your first few projects wondering why your pieces are all just slightly bigger than they should be.
A quick note on squares changing your life
I mentioned before that Mary got me a decent square for a birthday a while back, and I'll say it again here because this is really where it shows up. Before I had a good square, I was marking lines by eye, or with a scrap of wood I figured was probably straight, and then wondering why nothing on the shelf lined up square at the end. A good square doesn't fix your cutting. But it makes sure the line you're cutting to is actually a line worth following. Garbage in, garbage out, as they say.
An afternoon that had nothing to do with any of this
My neighbor Rodney came over once to borrow a drill, years ago now, and ended up staying about three hours. We were not marking shelves that day. We ended up building a birdhouse that neither one of us needed, just because the tools were out and the garage door was open and it was one of those afternoons where the next thing just kept seeming like a good idea. I think about that day whenever I'm rushing through a step like this one. Marking a line carefully isn't the exciting part of building a shelf. But it's the kind of unhurried, pay-attention work that the good afternoons in a shop are usually made of. You're not in a race. Nobody's timing you.
A caution here
Once your line is marked and your waste side is scribbled, keep your hand and your fingers well clear of where the saw's going to travel, even in this marking stage, because it's easy to get comfortable resting a hand right along the line while you double check it and then forget to move it when you pick up the saw. Get in the habit now of marking, then stepping back, then picking up the tool separately.
Before next time
Take a scrap piece of your pine and practice this whole sequence twice, point, square line, waste mark, before you touch your actual shelf pieces. It's not much for a habit to build, but it'll pay off the first time you're not thinking about it anymore.