Slow but accurate method.
Measure up window opening and mitre corners as necessary - make sure the
edging noze protrudes the thickness of a tile + adhesive beyond the window
reveal.
Stick up temporarily using pieces of masking tape and check that trim is
vertical/horizontal as appropriate.
When satisified smooth tile adhesive into hole cutouts on strip. Make sure
there are no blobs of adhesive left that would interfere with tile
placement.
When finished, re-check vertical/horizontal and trim position to make sure
none's moved.
Leave to set and then use trim as edge to tile to.
Personally I like to leave a grout joint between tile and trim, I believe
others push tile upto trim nozing.

Phil