Apply your file to the side of the pipe you plan to insert into your PVC fitting. Since you are inserting the filed pipe, file its outer edge.
File the pipe at a 45 degree angle, making sure the entire edge of the pipe is evenly filed. Remember to file gently, as filing too hard may break the file.
Apply adhesive to the filed edge of the pipe. This will allow your pipe to stick to the fitting better.
Insert the pipe into your fitting. If you need to, apply grease to the pipe you are inserting to make the process easier.