Although shoes should be comfortable from the moment they are tried on, the reality is that you often need to know how to stretch shoes here or there to make them more perfect. Unless they are custom made, which is the absolute ideal, it's the rare shoe that really will be just what you want it to be on the first wearing.
How to Stretch Shoes: Basics
Even before most people were buying ready-made shoes, shoe trees were used in between wearing to help shoes maintain their shape. If a person had bunions, they would add wedges to the tree in the right place, all to be sure that a shoe remained comfortable for the next time it was worn.
Many shoes require a "breaking in" period of adjustment, especially if they are made of real leather. Leather stretches and adjusts to fit the foot, so while the shoe may eventually be the most comfortable thing you own, it might start out giving you blisters. Hard leather boots in particular will be very heavy and painful when you start wearing them, and there's not much to do except just keep wearing them until they give. You want to be mindful of stretching leather too much at the outset, lest you stretch it too far.
With uncomfortable shoes, start off just using a shoe tree that fits the shoe exactly. You don't want to give the shoe a chance to bounce back to its pre-worn position. A home shoe tree can usually be adjusted to stretch, so start off slowly and see if it makes a difference on the next wearing.
If you have bunions, you definitely want to get a bit more aggressive. Most home shoe trees and stretchers will have attachments that stretch the toes to accommodate a bunion and you should use this overnight before the first wearing. If the bunion is particularly bad, you should use a leather softener to allow for even more stretch at the outset.
There are those who swear that if you don't know how to stretch shoes, your first port of call should be a stretching solution, bought in a spray. You simply spray on either the inside or outside of the shoe and then walk soon after - the idea is that it speeds up the natural breaking in process by mimicking the effect of foot perspiration.
There are two major problems with using this spray. One is that it can cause dyes to bleed or otherwise stain the fabric. The other concern is that the chemicals can be quite toxic and harsh. Ideally, you want to use the spray outdoors or in a well-ventilated area and allow it to dissipate completely before you wear the shoe. However, the feet are highly sensitive and it's unknown how much these chemicals might be absorbed by the skin or how long they linger. So you may want to try other methods of stretching shoes before resorting to the spray.
Consider a Professional
There are also those who say that the only people who really know how to stretch shoes properly are the better shoe repair professionals and it's best to spend a bit of money and go to them with a problematic shoe instead of trying to make it work yourself. This is especially so if you have bought high-quality shoes - there is no good reason to then try and adjust them at home and risk compromising them.
It should be noted that it's a shoe repair person and not a shoe seller to whom you want to take shoes that need stretching. These are two different specialties, and although a shoe seller may be quite knowledgeable and helpful, the way to tell the best seller is if they recommend that you take the shoes somewhere else to be stretched. It is another expenditure, of course, but it's worth it to end up with the perfect fit.