Do you mean...
Seal into frame?
Glass into seal?
Complete unit in vehicle?
I've just done mine and I'm sure others will agree, it's a ball ache. I didn't really have a trick as such.
Fitted the seal into the frame. Then fitted the glass which effectively pushed the seal into place. Kept the glass shut.
Then fitted the quarter into the van followed by the 3/4 piece which forced everything in.
Pushing pulling and whacks with a rubber mallet were needed and we got there in the end. It took about half a day to sort that alone.
Sorry I don't have any tips - just bloody mindedness and grim determI nation got ours in.
Good luck.