Design answer: underlay the wall and equip yourself with a spirit level, a ruler and a pencil
To paint your lines on the two walls you have chosen, I think it is best to underlay them entirely. Once dry, apply your matt paint over the entire surface of your wall. Then to visualize your wall, make a small sketch beforehand of what you want to achieve. To make your strips, you will need: a meter, a spirit level, mini paint rollers more commonly called rabbit feet, masking tape and a dry pencil. Draw your marks using the pencil, the meter, the spirit level and a ruler if you feel more comfortable. Then apply your masking tape to the outside of each lacquered strip to be painted from the ceiling to the floor. Take care to paint towards the inside of the strip so that the paint does not pass under the masking tape. Apply 2 coats and gently remove the masking tape as soon as you have painted so that it does not remain stuck to the paint.