Fix the larger concave lens at the end of the wider tube and the smaller convex lens at the end of the narrower tube. The width of each lens should fit the tube ends.
Glue the lenses to set them firmly in place. If the lenses are loose, cut a piece of Styrofoam that exactly fits the diameter of the tube. Carve out a space in the foam for the lens so that it holds the lens snugly. Glue the foam inside the tube and then fit the lens into it.
Insert the narrower tube into the wider one so it can rotate when you look through it. The smaller tube's rotation should be smooth. If it is loose, line it with Styrofoam to create a tighter fit. If it is too tight and rotation is difficult, use a slightly smaller tube.
Check through the lenses. The smaller lens is your eyepiece. When you view an object through the small end, the object should appear close and clear. If it isn't, rotate the tubes to realign the lenses to view the object clearly.