Empty the box that PiHut sent you the antenna and XIAO board in.
Poke a hole in the top
Feed the SMA pigtail through the hole and tighten the bolt down holding it in place.
Remove the LoRa antenna that came with the XIAO board - it is the skinnier one - and attach the small annoying end of the SMA pigtail to it. It is delicate, go slow and be careful
Put the XIAO board in a nifty mini breadboard and stick it to the bottom of the box
Screw the antenna onto to the SMA side of the pigtail that you attached to the lid of the box
Poke a hole on the side of the box, run the USB C end of the cable through it and plug it into the Xiao's USB C port
Load https://flasher.meshcore.co.uk/ in Chrome on your computer and plug in the other end of the USB cable.
Select Seeed Studio Xiao S3 WIO, then Repeater, check Erase device, and then Flash. You will be asked to select a Port, select one listing a USB modem or Serial and away you go. If everything works, you will see a lot of text scroll by.
Return to https://flasher.meshcore.co.uk/ and with the device still plugged in, select Repeater Setup, and fill out the information, such as name (usually something like bhd-landmark-handle), set a position (it doesn't have to be exact address, but it helps to know the general area of the repeater), and make sure UK Narrow is select.
Save and reboot the device if prompted
Unplug the box from your computer and plug it into your attic or wherever is best located and get it connected to power
In the MeshCore App, load Tools -> Discover Nearby Nodes -> Repeaters, and if you did everything right, you should see that new device and possibly other nearby ones as well.