Here is a short illustrated assembly step progression for a MightyHat R3. It’s important to do things in order to avoid misalignment of headers or incorrect soldering of the main Pi mating header.
First, the extra side header is soldered, if any. It’s easiest to do this part before any other headers. The hex columns are then attached and ready for the larger header. Note that you will need to cut down the black plastic screws since they are too long – they need to allow the long+short white hex nuts to mate.
We get the Pi ready, and insert the double length mating header all the way down into the Pi header.
Carefully insert the MightyHat through the header.
It’s very important to set the right height of the MightyHat before soldering the mating header. The plastic hex columns (~17mm height) are meant to help with this. The radio module will be just above the ethernet connector housing, but it should NOT touch it.
Once this is done and everything looks good, proceed to solder the header. You may only solder the pins that you actually use. The essential pins are the Power pins (pins 2,4,6), the serial pins (GPIO 14,15, aka pins 8,10), and the power control pins (GPIO 7,8 aka pins 24, 26). The rest of the pins can perhaps be left unsoldered.
And this is the finished result: