To add to the accepted answer, there are multiple types of kit for sale that help make this step easier. I've been making a lot more oat milk since we got this. To use it I just shake some unsoaked, regular rolled oats into the inner container (after sliding it into the right container), add water and go to town with a blender.
After that you use the weird stamper thing on the left in the picture to squeeze as much fluid as you can (muster the patience to bother with) out of the oat mush in the bottom of the container.

As for the accepted recipe, that looks about right for a base but don't be scared to experiment with it a bit.
I've found that tossing in a few hazelnuts or a pinch of salt can help bring out a more complex flavour.

To add to the accepted answer, there are multiple types of kit for sale that help make this process way easier. I've been making a lot more oat milk since we got this. To use it I just shake some unsoaked, regular rolled oats into the inner container (after sliding it into the right container), add water and go to town with a blender.
After that you use the weird stamper thing on the left in the picture to squeeze as much fluid as you can (muster the patience to bother with) out of the oat mush in the bottom of the container.

As for the accepted recipe, that looks about right for a base but don't be scared to experiment with it a bit.
I've found that tossing in a few hazelnuts or a pinch of salt can help bring out a more complex flavour.