The corner does the cutting. Round file inside out, keep 10-20% of the file diameter above the top plate. With full chisel chain you can use a little down angle, say 10*, on the file handle to get into the corner. Set the rakers at .030 below the top plate. Lay a hacksaw blade across the top plate and look through the gap to the light to judge the raker depth.
Another trick is to put the saw in a vice held by the bar and loosen the chain slightly Use the brake to stop the chain if it wants to move. Work in the same spot with good light. If I have some damaged teeth, I use a crescent wrench to measure the shortest tooth and then file them all back to that size. The round files dont last, lay a rag under where your working to see what your removing. Twist the file with your wrist as your pushing the file through the tooth and cutting the angle. Dont drag the file back over the tooth.
A couple more tries and you will get it.
Another trick is to put the saw in a vice held by the bar and loosen the chain slightly Use the brake to stop the chain if it wants to move. Work in the same spot with good light. If I have some damaged teeth, I use a crescent wrench to measure the shortest tooth and then file them all back to that size. The round files dont last, lay a rag under where your working to see what your removing. Twist the file with your wrist as your pushing the file through the tooth and cutting the angle. Dont drag the file back over the tooth.
A couple more tries and you will get it.