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If you are putting molding in a 4-wall room and started with a piece with two 90 angles, you will have to make one piece that has two 45 angles. The 45 angles will need to be opposite angles. Be sure to leave an extra inch or two at first to make sure everything fits. A slightly too long piece will actually make the entire construction a little bit more snug and prevent cracks from forming as the house settles. For a 4-wall room you should end up with one piece that has two 90 ends, two pieces that each have a 90 end and a 45 end, and one piece that has two opposite 45 ends. Run adhesive along the flat faces that touch the wall and ceiling. Also put adhesive on the parts of the molding that connect with another piece. An extra pair of hands will be a great help when installing the molding, especially for the longer pieces. Push the end of the first piece firmly into the first corner. Use finishing nails to secure the molding while the adhesive sets. Use a nail punch to make sure the nails go under the surface of the wood. This will allow them to be painted over. Attach the rest of the molding pieces, using caulk to fill in gaps as you go.
Repeat the steps above for the remaining pieces of molding. Attach the molding.