You will need
  • corner
  • -box cutter
  • -line
  • ceiling plinth
Measure the corner of your room. To do so, attach the area to the top of the wall in the corner near the ceiling. Should be approximately 90 degrees. If you have a smaller value, this means that the corners of the room are uneven, trim and baseboard will be a little difficult.
Take the ceiling tiles and attach the line at an angle of 45 degrees and a bold hand with a utility knife cut the excess part of the floor. With the second cut, do the same. For baseboards of a dense material (wood, plastic) use a hacksaw or jigsaw.
Try cut off part of the ceiling moldings to the corner of the room, if it fits and the angle turned to the desired degree, feel free to apply glue and attach them to the wall.
If the corner is uneven, measure its degree. Then divide in half, this will be the desired angle of the cut, you need to do on the baseboard. Example: the corner of the room is 80 degrees, so you have to cut off the angle of 40 degrees with the floor.