RICCAR - My Sewing Machine Manual - Pg 1
Sewing leather on a sewing machine requires a different technique from
sewing fabric or normal materials you would sew on a sewing machine.
Unlike fabric, you cannot just put a piece of leather under the machines
foot and hit the power and start sewing.  This is due to the density of
the leather, which is one of the reasons for using a leather needle,
which has a special piercing tip specially designed to penetrate the
tough fibers in leather. The technique for sewing leather is as follows:
Make sure you are using a leather needle in the machine, these come in
various sizes to accomadate various thread thickness. I myself like to
use a size 18 needle with commercial nylon sewing thread, size 69 or
size 92. These are ideal size threads for most leather sewing work. You
always want to use a nylon or poly thread with leather, as cotton thread
will dissolve over time due to chemicals used in the leather's tanning
processes. Once you are ready to sew, make sure you have pushed down the
presser bar spring on the top front of the machine to give proper
feeding pressure to the sewing machine foot. Using the machine's hand
wheel, turn it towards you and keep turning it to make the first few
stitches in your leather. Then, continue turning the handwheel while
slowly starting to press on your foot control pedal, adding some power
to the motor. With a bit of practice you will feel when the motor starts
to take over and do the sewing. Once the machine is doing the sewing, it
is best to sew at a fairly slow speed, as unless you are sewing thin
leather, you will find you have the best control for detail sewing at a
slower speed. It is important to use the handwheel, as without that, the
needle does not have enough space to drive itself into the leather, as
it is confined into a small area of travel. This technique is used even
on the full size industrial sewing machines for starting stitches in
leather. Once you get the technique and the "feel" of using the
handwheel along with the motor, sewing leather will become a snap.