Updated: Jun 8, 2022
You start an oil painting from the ground up, quite literally:
The ground is the first layer, that will be hidden in the final painting, so why bother with it?
Well, because it is vital for achieving reliable results. The ground sets the drying time of your oils, it sets the rheology (how the paint flows over the surface), it sets the basics of any translucence if you want optical to achieve optical effects, for example, if you are using glazes.
So, how do you prepare this ground thing? Well, you could boil rabbit skins like the old masters did. Or you can just add chalk and / or marble dust (depending on the effects you are trying to achieve) into white acrylic (or any other colour to be honest) in a proportion of 1 to 3 (depending on how quickly you want your oils to dry and what rheology you want) and then slap it onto whatever you are going to paint on with the biggest brush you can find: canvas, MDF boards, plywood, an old barn door... You name it, you can paint on it. It just needs to be durable, but only if that matters to you (for example, if you want to sell the painting...).
When laying down the ground, there is no need to neat: The more serendipity the better. In fact, sometimes the ground can be an art form in its own right!
There is so much more to say about grounds, but don't take my word for it. Instead, do a course with Martin Kinnear. Your oil painting will never be the same again.
The above became the below: