In a perfect world, every pair of shoes that we love would be comfortable right out of the box and fit our feet like a glove. As we know, it is not a perfect world. There are many reasons why shoes don't fit properly, ranging from stretching to a difference in foot size to the construction of the shoe or even just the style of shoe. Adding inserts or supports to a shoe can help the fit.
Ask a shoe repair store or the shoe store where you bought them to stretch or soften your shoes if they are too tight. Shoes can be stretched by putting them on a wooden last for a while or there may be topical products that can be applied to soften the shoe leather.
Glue two cork insoles and insert into your shoe if your foot slides around inside the shoe when you walk.
Insert a thin foam insole into the shoe to take up some room if your shoe feels too big.
Press an adhesive felt tongue pad to the tongue of your shoe to help keep your foot in place if the shoe is too big.
Adhere a heel grip, a U-shaped foam piece with adhesive backing, to the heel of your shoe if your slip-ons gap. This will help keep your heel in place and help prevent blisters from your foot rubbing against your shoe.
Exchange your shoes for a smaller or larger size if possible. If your feet are drastically different in size, buy two pairs of shoes, one in each size.