Infusers come in several varieties. The simplest is the metal tea ball. These balls are typically round although some come in decorative shapes. They have either mesh sides or small holes in the sides. The ball prevents the tea from floating loosely in the water but still allows the flavor to steep out. Most tea ball infuser have a chain that you use in the same way you would the string on a tea bag.
Although usually reserved for coffee, a French press also works well for making loose leaf tea. I have a glass version that resembles a tea pot and makes and entire pot of tea at once, but smaller single cup varieties are also available. There are several designs of infuser pot available. Some even resemble traditional tea pots while others have a more decorative shape.
There is a lot of room to play around when making and blending your own loose leaf tea, but in general 1 teaspoon of loose leaf tea per cup of water is recommended. The tea leaves swell as they rehydrate in the water so you don't want to overfill the infuser. Fill the infuser with the tea leaves, then place it inside the tea cup as you would a tea bag. For pots, first fill the infuser chamber with tea then add the hot water to the pot.