Don’t view scrap wood as just leftovers. For beginners, the right scrap is where you safely make mistakes: take your practice marks, run test cuts, drill pilot holes, find the right clamp pressure, and experience for yourself the effects of sandpaper. The offcut, if chosen carefully, may have more to teach you than a larger board will.
Choose a size of scrap that you can hold on the bench top securely. Small pieces are difficult to control, because often they require holding them with one hand very close to the work area and the cutting edge of a saw, bit, or sanding disc. Your practice piece should be large enough for you to mark a line, attach the necessary clamp, and position your fingers clear of your tool. Softwood boards and plywood will work well for you at first because hard woods often fight the saw and split more easily when you drive a screw.
The face of the board that you are going to use should look over before you start using that piece. The board should not show deep cracks, loose knots, too much bowing, or splintering edges that will make it difficult to tell if a bad cut or bad clamp pressure was due to a piece that might not have been in good shape at the beginning. That does not mean that you should only pick practice pieces that look brand-new. Some rough spots can serve as a good reminder, since that is what you can expect wood sometimes to do, but the piece should not be so damaged that you can’t measure, mark, cut, or sand it.
Scrap also can teach you about the direction of wood grain. Rub your hand over the board, and look at the grain lines. The grain changes direction or comes out at a slope to the edge, which affects the sanding feel in that area more than other areas on that board. For this reason, you might want a couple of boards just for this exercise: take a pencil line on the piece, sand one of the squares with a medium or coarse-grit sandpaper, sand another part with a finer-grit sandpaper and check the results side by side in good lighting so that you can really see the difference.
Choose a piece that has one really straight edge for cutting practice. You need the straightest edge possible so you can place the tape measure against it and set your square and line for cutting with your saw. If the board’s other two edges have some taper to them, you will have trouble even if your line seems to be correct. Place the square against the straightest edge and use your straight edge to draw across the board. If it is a perfect cut, you should be able to follow the line all the way down the board’s edge. This is an exercise to train your layout and to help you think through your measuring before picking up the saw.
For making holes and for using fasteners, pick pieces of scrap that aren’t too thin and don’t already show cracking close to an edge. There has to be enough wood around where the screw will pass in for it to hold, but you don’t want the screw breaking out the side of that piece. Put a pencil mark on your board for where you need your pilot hole to go and then check how close you are to the edge or the end of the board edge. If that screw is going to be too close, shift over the mark, and try another drill pilot. This is one of those small decisions that will be easier to test without wasting parts in a project piece.
Rather than picking just one board, choose a few different pieces of scrap for different exercises. One might be good for practicing marking lines, another for practicing cuts, another for pilot holes, and another for sanding practice. If you find that writing notes on your scrap wood is helpful, you could just write the grit number, drill bit size, waste edge, or clamp on the board, so that later you can look back at it and see what your hands have tried. A good scrap piece is one that will not make learning more complicated, but one in which you can easily handle the wood in a safe manner so you can follow the line with your pencil, be able to see the grain in the piece, and see the difference after a test. Before you pick a board for any practice, ask yourself what it is that you want to learn: If it’s measuring, you need a straight edge for reference. If it’s about sanding, you need an area of the board that shows the wood grain. If it’s for drilling, you need a board thick enough for making the pilot hole cleanly. The wood you practice on will determine what kind of lesson you will be learning.