Not difficult to install if you can open your wing-to-body gap enough to slip them in there. Need about a 3/32" gap to accomodate the fastening lip on both guards plus the tabs for the wing bead.
Only necessary to undo the forward foot or two of the bead and carefully bring it forward to allow the guards to slip in.
I recently had to replace one side because of a parking lot curb mishap. I initially ordered from TRF but the shape of the small guard seemed unsuitable & likely to need tweaking to fit my car. Here is a side-by-side pic of the TRF small guard on the left and my old guard on the right (note the 'dip' along the front edge of the TRF guard):
They took them back no problem and I ended up getting them from Moss. My prior set (one side of which met the curb) was from Vicky Brit and they were similar to the Moss supplied parts but perhaps a bit shinier.