To get the best fit, you’ll probably need to remove the shoe’s existing insole and replace it with the one you purchased. If you buy an insole that requires trimming, trace the shoe’s original insole onto your replacement and cut it to size. Most supportive insoles should have enough substance not to slide, but if they are moving too much for you, you can permanently attach them to the shoe with an adhesive like cobbler’s cement. If you do this, remember, you won’t be able to move them to other shoes.