I know how to put hammers in a piano but trying to explain it may not
be as easy.  So here goes my experience.

Most new hammer sets have a few extra hammers in them.  Number all the
hammers first.  I start with the treble end which seems to be the most
critical place to start due to the short length of the string.  I would
suggest placing the action in the piano.  I usually have the piano
tuned first before any hammers are replaced.  You can place this treble
hammer in the piano and move it up and down on the string till you hear
the "sweat spot" on the string.  Grand pianos often have a screw in the
key-blocks   (also known as a cheek block)  to adjust a piano action for
this "sweat" spot.  This is when the note is pure and clean sounding.

I often glue the hammer shank in first and use a spare hammer with a
hole drilled all the way through the hammer so you can move it up and
down.  The hammer will hit the string at a point very close to the top
of the string or that point nearest the upper bridge.  I then do the
Hammer at the end of that section the same way.  I take a straight
piece of wood about 3/8" square and use a rubber band to hold it on the
bottom of these two hammers at their widest point looking at the hammer
from the side.  I then take another straight piece of wood about the
same size (3/8") and rubber band it to the bottom of the hammer tail.

With these two straight pieces the rest of the hammers now have a guide
to rest on.  I usually put in a hammer in the right end and then one in
the left so when I get to the middle they better be straight or I
goofed up.   I use a machinist square to make sure the hammers hit the
strings square.  I do the rest of the piano this same way.  You should
have a nice straight set of hammers when finished.

You can always look at another piano  that has never been touched and
notice how straight the hammers are from the factory.  The exception to
this would be three or four hammers at the wound- tenor section break.
On rare occasions these hammers may go up at a right angle a bit but
this is very rare and I have only seen it in 42" to 45" console pianos
made in the last thirty years.  f One tip is "do not get the hammers on
the shanks to tight".  They have to be snug and not loose.  When they
are too tight the hammer will not stay put as the moisture in the glue
will make is go left or right.  By all means when you are finished the
hammer must meet the strings square.  New hammers when you look at them
close up will have a "u" shape or   "cup" in them.  These new hammers
have to be sanded to remove the cup.    Of course the  next step will
be a lesson in voicing.

I personally believe there are many fine hammers being made new today.
I have not been that happy with the way some of these hammers are
drilled for the hammer shanks, so therefore you might take a close look
at your work only to discover poorly drilled hammer shank holes.  If
you decide to drill your own hammer shank holes then use a short drill
bit in your drill press or milling machine with a machinist vice.
These are expensive but once you own one...well you will love it.

Don Teach, Shreveport Music Co
1610 E. Bert Kouns, Shreveport,  LA  71105