Raid your sock drawer for a pair of thick socks. If your socks are not particularly thick, put on two pairs of medium thickness or snag a bulky pair from your sweetie's sock drawer.
Spray or squeeze a small amount of liquid shoe stretch for leather on the inside of the boot. Don't get carried away and apply it everywhere, you don't want to stretch out the entire boot after all, so only spray it where you need the extra width in the foot of the boot or at the calves.
Pull your boots over your thickly socked feet. If you're pushing and pulling but the bulk of the socks aren't allowing your foot to slide easily into the shoe, consider putting a plastic bag over the sock. This will make it easier for your feet to slide on in.
Walk around your house with your boots on. How long you'll need to leave them on will depend on the brand of liquid stretcher that you're using, so follow those instructions and leave your boots on only as long as recommended. If time allows, this a great time to try out your new boots with a few outfits in your closet.
Remove the boots and place them in a location that's away from a heat source. This way it can dry naturally if the shoe is still damp from the liquid stretch.
Try on your boots again, with your regular socks or stockings this time, and see if you've gained the additional width that you need. If not, sock up and give it another try.