Allow your laminate flooring to acclimatize to the temperature and moisture levels in your home before installing it. Seventy-two hours typically suffices for most floor types, but check your flooring's installation instructions.
Install your flooring in your first room as per the manufacturer's instructions. You'll need to prepare your base floor, fit an underlay and use expansion spacers to create a gap between your flooring and your walls.
Measure the height your flooring reaches on the architrave of the door frame that leads to your second room when you're ready to start laying through.
Draw a line around your door frame architrave into your second room and then remove the lower section with a hand saw.
Measure the angle at which you'll need to cut your flooring planks to fit under your door frame's architrave and into your next room with a tape measure. Remember to account for the expansion gap recommended by your flooring's manufacturer.
Cut your flooring to the measurements you've taken using a circular saw.
Slot your flooring into place under your door frame's architrave. Use laminate wood glue, a knocking block and hammer, and a pull bar to secure your planks into place. You can then continue to lay your flooring in your second room.
Fit a door threshold between your two rooms if required. The method for doing so will depend on the type of threshold you've purchased.