If you are Replacing old door with new
door you may need to cut down the door to fit the old opening . It
is best if you do this before you mortise the hinge or drill
for the knob because it is nice to trim both sides evenly .
But don't count on the opening to be square or even round so measure
carefully before cutting the new door and also double check the inside
outside , top and bottoms so you are cutting the right edge from
the right side .
Once you have figured out where the door needs to be cut , find a place
to cut the door . If you have a shop with a table saw you can use
that and you probably don't need to read this ..
but if your a home owner you will still need saws and tools to cut
the door . At the minimum you will need a circular (skill) saw and
a sanding block or a belt sander , a block plane can be handy too , as
well as all the tools for installing the hardware needed later .
A table saw with a wide table is best if you need to cut the edge of
the door , but you can also cut the edge with a skill saw ,
with or without a guide or trim it with a good plane which you should use
to smooth the saw cut and ease the corners before sanding it .
check that the bottom of the saw is smooth so you don't scratch the
door and that the blade is square to the foot .
If it is an exterior door the bottom should be tapered to the amount
in the thresholds instructions or 5 to 7 degrees with the short side towards
the threshold .