how do you enter the astral plane from the etheric?

By eating cheese before bedtime.

You need at least a level 9 cleric or druid.

You need a quantum rocket capable or traversing non-euclidean dimensions.

Very carefully, hopefully.

Not knowing exactly where you are helps. You can achieve this by spinning, zooming upwards out of the atmosphere, or momentarily filling your visual field with a patterned surface and nothing else. Exiting your room via a portal (such as a window or mirror) also helps, as does trying to enter a picture. Just staying out for a long time tends to gravitate you towards the astral as well.