The IKEA TRÅDFRI switch instructions are pretty simple (but ignore the direct pairing of a remote to the plug):
- Plug in where it is going to be used (ideally so it calculates the mesh neighbours from the start - if this doesn’t work, try next to the co-ordinator).
- Turn on
- Set your HASS Zigbee automation into
(+ Add New Device). - Use a paper clip to press the reset button
- Fiddle randomly with the reset button trying one press, one long press, three presses, five presses, trying to get the white light to flash.
The hard bit is getting to press the button (hole is a bit small) then the white LED to flash - like the 5-button remotes, random button presses and power cycles sometimes work better than a structured approach.
Once adopted, TRÅDFRI switches just work.
If this helps, 
this post!