Unscrew the stays for the window and the locking plates but mark where they should go back , use masking tape and stick to the van wall near the window rubber.
Remove the infill strip.
Remove all 100 or so staples you will find under the infill strip.
Pull the old seal away from the window.
Clean off any old mastic and repair any damp issues.
Starting at the centre bottom edge, fit the new rubber making sure it sits tight against the frame, especially in corners.
There should be mastic already present in the new rubber which seals the outside of the rubber to the van.
Make a good tight joint where the seal meets up in the centre bottom edge. Use a smidgin of silicone sealant on the mating edges to seal the join.
Put back the staples with new.
Refit the infill strip
Refit the locking plates and window stays in the locations you have marked.