You can put them just about anywhere you like.
Depends on the type of bells you have as some are made to go a certain place,
I have these saddle chimes, you take off the turrets on the saddle then put the bells on then put the rein turrets back on. saddle chimes.jpg
The typical sleigh bells go around the horses girth, usually in front of the saddle, and on the outside of the shafts. sleigh bells.jpg
These are shaft bells, they go under the shafts, somewhere behind the saddle. These are removable, if they were on a sleigh they would most likely be permanent. shaft chimes.jpg
But If you just have a strap or two with a few bells on it you could hang them just about anywhere.