Is it ok to make a small homemade generator, the smaller the better, output power is around 24-36V?

Yes.

1: Make a wooden water wheel.

2: Nail the waterwheel shaft with nails, but it must be able to rotate.

3: Nail a copper plate to the tree.

4: Make a parenthesis.

5: Make a horseshoe-shaped magnet on the support.

6: Connect the wires to the magnet and the nail shank.

Full (If the power is too high, reduce the water flow; if the power is too low, do the opposite.