here are some pics of the pHATs soldered the lazy way. By lazy, I mean that only some of the pins were soldered. For simplicity (or added strength), sometimes I included some pins unnecessary for functionality.
a basic rule of thumb, if you’re not sure what is needed, is to solder 6 pins (2x3) at both ends, then check http://pinout.xyz for any additional pins needed for operation. It’s sometimes possible to tell what pins are in use by examining the circuit board (on both sides!), and if you’d like you can have a mini-game of guessing before you check on the pinout site!
why do this? well, besides saving time (and solder!) it will also help you concentrate on fewer pins to get right. It may also help you visualise at a glance what boards may be compatible when used in tandem (with some degree of accuracy)… whatever the reason, if nothing else, you’ll learn a few things in the process!
for the Explorer pHAT, I recommend using an ‘extra tall’ header as shown, and solder all of the pins (i.e more than is shown), as this is a Pi add-on that lends itself to additional GPIO control.