look at the moon! but please don't try to look at the sun...
check that you have two convex lenses (convex lenses should bulge out at their center). if i didn't screw up, your two lenses should be of different sizes. the smaller is the eyepiece lens, with a focal length of 20mm. the larger is the objective lens, with a focal length of 300mm. if needed, wipe both lenses clean using a microfiber cloth.
carefully slide the lenses into their respective optical tube. optional: line the inside with matte black paper to reduce stray reflections.
in our keplerian telescope, the lenses are placed apart by the sum of their focal lengths. 20mm + 300mm = 320mm. use any connector of your choice between the two optical tubes. with a ruler, measure the distance between the lenses and adjust the tube length accordingly.
look through the telescope at a distant object during the day. adjust the focus by sliding the optical tubes relative to each other. never ever point it at the sun!
find out what your telescope's magnification is and what the number represents. design and build a stable base for your telescope so it doesn't slip around in your hands. research other types of telescopes, maybe find one that works even better and try building it!
prove your allegiance to the cause of astronomy by answering some boring math questions about focal lengths. you will need to know what a focal length is, why it matters, and the difference between galilean and keplerian telescopes.