Using a sharp knife slice lemon in half across the equator -- that means the middle and not from end to end. Be careful: the knife is sharp. If you're not allowed to use the sharp knives, find an adult who is allowed.
Using your hand or a citrus squeezer, over one of the bowls squeeze one half of the lemon until all the juice has come out. (If using a squeezer, the cut side goes into the concave side of the squeezer.) If some pulp and seed come out, that's fine. If using a juicer, put the cut side down on the pointy bit and press and turn to get all the juice out.
Repeat with the other half of the lemon.
If you need more lemon juice, repeat.
Once done, with the strainer over the second bowl pour the lemon juice into the strainer to capture the seeds and pulp.