Fitting a shoulder rest correctly is a crucial element in achieving a comfortable and uninhibited playing setup.
Most shoulder rests have a wider curved part and a thinner part. Place the wider side of the rest on the bass side of the instrument (G string on the violin, C string on the viola).
Starting on the bass side, hook the c-shapes of the feet of the shoulder rest on to the edge of the violin, at the widest part of the bout - the 9′o clock position. Then slide the other feet onto the outside of the violin stopping at around the 3’o clock position. If the shoulder rest is too tight or too loose, most have adjustment built in, so try lengthening or shortening the rest by adjusting as necessary, in order to achieve a snug, secure fit which is not overly tight.
As a general rule of thumb, if you have narrower shoulders, you may find a position closer to the endpin more comfortable, whereas those with wider shoulders may prefer a shoulder rest fitting closer to the C-bouts.