Electrical – How do i connect multiple ground wires? There are 3 green and 1 bare wire on the light fixture

The instructions for the fixture are only correct for a metal box. If a metal box was used, the box itself would (should) be grounded. The bracket that holds the light would then be connected to the box, which would make the bracket grounded. Finally the ground wire from the fixture would attach to the bracket, grounding the fixture.

In the case of a plastic box, the box is not grounded. All ground connections must be made by connecting the ground wires together. So in your situation you are correct, you'll connect the ground from the supply to the ground from the fixture using a twist-on connector (or other approved connection).

If the old fixture worked, at least one of the white wires is hot.

The easiest way to check which is hot is to use a non-contact tester. If the fixture is already removed, leave the two wires bundled together with a wire nut over the end. Put a wire nut on the single white wire. Turn the breaker back on. Only one of those should register on tester. That is the hot side. The other should not register. That is the neutral.

Once you know which wire is hot, mark it with black tape or a permanent marker.

If the the fixture is still attached, you can do the same thing with the fixture hanging down. Once you know what is what, turn the breaker back off before wiring.

The black wire goes to the hot wire, the white to the neutral. The ground wire should be attached to a green or bare wire if there is one in the box, or to the box itself if it is metal. If not, just cap it.