Externally the trim legs each measure approx 58mm and 7mm wide

The trim is 3 dimensional, being approx 7mm deep,

The reverse has a slot on each leg, measuring 45mm long x 4mm.  This length is the fabric section width (you can make the fabric wider if you want a gathered effect).  The fabric will pass through the slot and then be sewn back on itself, you'll do this on both sides and the trim has a little lip at the bottom to cover the join.

Absolutely exquisite.