The simplest type of telescope consists of two convex (magnifying) lenses. We will call the far one the objective, and the close one the eyepiece. Each of the lenses has a focal length, which you can determine by using it to focus sunlight down to a point and measuring the distance from the focal point to the spot, or with a number of other methods that you can probably find online. The magnification (how much larger things appear) is determined by the ratio of the focal lengths of the objective and eyepiece lenses- ideally, for a large magnification, you want the objective to have a large focal length and the eyepiece to have a very small focal length.The other thing to consider is the sizes of the lenses. The eyepiece should be small so that it fits your eye well, and the objective should be large so that it catches a lot of light- the image will appear dim of the objective lens is small.You can therefore construct a simple inverting telescope with a large, low-power magnifying glass lens and a small, high-power lens such as a loupe.