To mount the stem, you'll want to first find your middle line on the top of the base. Use a measuring tape and a square to draw a light line up the middle as pictured. In hindsight I'd say the screws were more trouble than they benefit, and I'd suggest following the glue only instructions below.

If using screws: drill, from the top, two holes slightly larger than your screw thread diameter (you don't want the screw binding in the base) on the line, not too close to the edges. Flip the base over, then drill a shallow larger hole on the small hole, wider than your screw heads (so the screw heads don't protrude)

Whether or not you use screws, apply carpenters glue (not too much, you don't want it squishing out the sides) to the base of the stem before positioning it centred on the line.

If you're using screws, screw up from the bottom, through the base, into the stem. Hold the stem firmly in your other hand or clamp it so it doesn't move off the line. Hardwood is not the easiest to screw into, so you may want to predrill the stem with narrow pilot holes (you'll know where if you poke it with the screws coming up through the holes). I did this and it made the process much easier, as the stem is less likely to move around as you try and get the screws started.

Make it tight so the two pieces fit snug together but don't really crank it (it is not necessary, the glue will make things very solid) - you don't want to split your stem. if you're using a cordless drill with an adjustable clutch, keep the setting very low and up it if it slips too early.

If you use glue only, clamp from the top notch where your headphones will go to all the way under your workbench as in the picture, with the base sitting flat on the workbench. Don't clamp too tight - you could break your stem.

Congratulations! The hard part is over. Now to make it look great. If using glue only on this step, be sure to let it dry before proceeding.