Arrows show pocket hole locations
I don't have a pocket hole jig, so the next best approach was to drill vertically through the 4" deep chair frame. However, the screws I have aren't long enough to go all the way through, so I needed to drill a large countersunk hole, bigger than the screw head, and drill a smaller concentric hole through which the screw shaft would pass and finally drill a small tap hole in the chair seat. Drill stops make it easy to ensure the right depth of the larger counter sink hole.
Screws and tools required: drill bits and drill stops
Most drill bits don't have pilot bits on the end, so your best bet is to drill the small hole first and then drill the countersink using the first hole as a guide. Or, use a Forstner bit with a spike on the tip, or as I did, a bit with a pilot tip, and use the small center hole these bits create as a guide to drill the smaller hole. Position the chair seat, screw the screw into the chair wood a little, to create a mark, then drill the tap hole into the chair seat. Now fasten the two parts together. Below is the countersink hole offset from the pocket hole.
Pocket hole, top; countersink hole, bottom