Slants are test tubes containing agar that was allowed to cool and harden while on an angle. Slants are most often used for storage of an isolated strain. Being much smaller in diameter than petri dishes, and with a much deeper agar layer, they hold moisture better and provide plenty of food for the relatively small surface area. They also use less agar than petri dishes so that several slants can be made for the same amount of agar as one dish uses. The test tube should generally be at least 13mm wide, and may be as wide as 25mm. Wider test tubes are easier to work with, but use more agar. Shorter tubes are considered better, with a length of 100mm being considered ideal. Again, longer tubes use more agar. As agar is one of the primary recurring expenses, smaller test tubes are cheaper to use in the long run. The smaller tubes are cheaper to purchase as well. Slants have been successfully made from screw top test tubes, straight edged biological type test tubes, and flared or flanged chemical test tubes. Use whatever makes you feel comfortable. Essentially what is done is to carefully fill 1/3 of the tube with agar. It is possible to fill sterilized tubes with sterile agar, but perhaps an easier way is to mix the agar and fill the slants immediately, then sterilize the entire prepared slant. The slant top must be sealed in some way. If the tube came with a screw cap, that should be sufficient. Just make sure any screw caps are loose, before sterilizing. This will make sure none of your tubes crack. Otherwise plug the end with non-absorbent cotton and cover with aluminum foil. Applying the caps or plugs before autoclaving helps to ensure the sterility of the whole. After you have a filled and sterile slant you need to set it at a 15-30 degree angle to harden. A good rule of thumb is if the top of the agar approaches the top of the slant, you have about the right angle.
Here are a couple of links to photos of test tubes. These are approximately 16mm by 125mm, and are a very convenient size to use. They have been successfully colonated with mycelium of a Lentinus edodes culture. That is the common shittake mushroom.
One Slant Second Slant