When laying the plastic underlay, which protects the boards from moisture, check with the manufacturer to see what thickness of plastic they recommend.
Lay it over the whole floor and overlap joints by 200mm and secure it carefully with plenty of duct tape.
At the junctions of the wall, fold it up just enough so that when the skirting goes back on it’ll cover the plastic.
A straight edge a utility knife are the perfect tools for this job.
Foam underlay provides cushioning for the boards and absorbs some sound. It needs to run the same way as the boards, but you’ll need to check this is the same for your brand of boards.
It’s important the first row of boards is running perfectly straight. To check if your wall is perfectly straight, use a stringline.
If it’s not, you’ll need to scribe the first row of boards to fit the line of the wall.
Before cutting the first row of boards, measure the width of the room to ensure the last row of boards isn’t 50mm or less.
It’s a good idea to put some sealant between interlocking boards to protect the flooring from any future spills, like coffee or juice.