There should be instuctions on the laminate. But anyway, you need to lay the underlay and a piece of laminate (upside down) near the door case, then using a saw (flat to the laminate) cut the bottom of the door casing away. Then you need to cut the laminate so that you can fit it under that cut out.
When that's not possible, just cut it as close as possible and run a bead of decorators caulk around the edge.
if you cannot undercut the frame i would always leave a 6mm gap around door frames otherwise your causing a pinch point that may or may not catch you out
and the worst thing is if you do have trouble around a tight door frame its the most difficult expansion problem to solve
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