Well.... the best way would be to get the shop to come and fit it! Most will, I work with fusion saddles and I know they go any where in the uk to fit a saddle (all thought they are not cheap) but if your doing it your self I can give a few tips
You need to be able to get 3/4 fingers between the pommel and the wither of your horse,
Run your fingers down the spine of your horse then when you sit the saddle on his back there should be good space ALL the way from front to back ie: not pinching his spine,
And make sure the saddle sits close to his sides not "flapping about"
Lastly NEVER try to fit a saddle with a numnah. I know we all use them but its actually best for the saddle and horse not to use one EVER! But if you DO use one make sure when you tack him up you pull the numnah right up in to the saddle gullet, if it lays on your horses back and wither,and you put the saddle on it, then that un-does any good work you have done buying the right saddle! As its putting the pressure straight back on the spine and wither!
I've TRIED to make that short and hope it helps!!!