Start at the bottom left.
Cut the first plate straight on the left.
Fasten the first plate with countersunk head screws. Do not fully tighten the screws.
Slide the next plate into the first fixed plate almost to the screws. Then tighten them. Slide the plates together tightly.
Always continue this way. In the case of the wall connection on the right, cut off your plate straight.
With the rest piece (> 10cm) you start again left. To work along the wall from the bottom left to the top right.
Leave to floor, ceiling and sideways min. 1 cm of air (at approx. 2.4 m of room height), so that the wood can move a little when the moisture is balanced.