Seems like an easy thing, but there’s actually a trick to it that a lot of DIY’ers don’t know. It’s something that gets done a lot, so I thought it would be a nice first subject in this group.
Many (if not most) people will choose a drill bit smaller than the screw, drill a hole in each piece with this same drill bit, then screw the wood together. This usually works. However, it provides little or no clamping force; over time if subjected to any strain whatsoever, the screw will start to wobble, and work itself out of the second piece or break in the center.
The proper method is as follows:
1. Drill a hole in the first piece, using a drill bit the same thickness as the OUTSIDE dimension of the screw. (For example, most common screws that I use, I will use a 3/16” bit.) The screw should move freely in/out of this hole.
2. Dry fit this piece to the second piece. Put the screw in and tap it with a hammer to make a mark on the second piece.
(Alternate method: Hold or clamp the pieces together, drill smaller hole through both pieces, then re-drill the first piece with larger bit.)
3. Drill a hole in the second piece, using a drill bit the same thickness as the INSIDE dimension of the screw. (For example, most common screws that I use, I will use a 1/8” bit.) I buy a lot of 1/8” bits because they break easily… Like when I set the drill down and then step on it :(
Tip: If you do a lot of this kind of work, there’s nothing that says you have to make do with a single drill. If you have two, you can go back and forth between the two sizes of bit without switching them out each time. Yes, they do make quick-change bits but I dislike them because they seem to break a lot easier.
4. If necessary, use a countersink bit to countersink the screw. (Softer wood may not need this step, or rougher work.)
5. If you are expecting this to be permanent, apply wood glue.
6. Screw the wood together. Doing it this way will clamp the pieces securely together and it should hold up to a lot more strain over time.
Final note: There are times to use screws, and times to use nails. (Sometimes I even use both!) Nails are preferable to screws when there are shear forces involved (for example, attaching stair risers to a header), as they are stronger. The drawback to nails is that they have negligible clamping force; smooth nails have the least, and galvanized have more. I’ve been known to attach floor joists with screws first and then add joist-hanger nails; this prevents the boards from moving apart while I am hammering.