There are two types of glides below. The installation method between each of them is different. One needs to be hit in with force. The other uses an adhesive.
The friction fit glides go in with force by hitting them in your leg cups with a rubber hammer or mallet. This type is most likely what came installed on your furniture. They only work on furniture cups with exact inside diameters of 1-1/2" or 1-1/4". They come in two styles. Standard and Deluxe. The standard style has been around the longest and is what most people know. The newer Deluxe style is thicker, longer-lasting, and easier to install.
Forever Glides are the easiest to install. They use a two-part system. A flexible sticky piece of neoprene foam goes in first. Then a long-lasting plastic glide made from Delrin sticks on the foam. This combination lets the protective glide flex and move. The movement provides some self-leveling. Forever Glides are perfect for furniture cups when the inside diameter is smaller or larger than standard. Exact cup measurements are not needed for these to work. Just close enough to 1-1/2" or 1-1/4".