One of the marvels of painting (or drawing) is that you always begin again from nothing. The openness of the blank page is very humbling. With each picture there is the potential of relearning your vocation from the ground up.
When I draw something – it can be anything – one of the attractions for me is that you can begin the thing from any beginning. A landscape could begin with the sky, or with a point in the distance where a path disappears, or with a particular limb of a particular tree. You can start without things at all, you could decide to work from the top of the image to the bottom if you wanted – or from the middle to the sides, from left to right, from an all-over-ness to details, or you can begin with details and work out toward the large forms.
Various art schools will tell you that any of these possibilities are wrong, but you always have the option of ignoring the advice to experience for yourself what happens when you try the forbidden procedure. This is very different from doing surgery or flying an airplane. The surgeon and the pilot cannot arbitrarily decide to reinvent their entire methods. But the artist can. Artists are wise to keenly observe the fact that nothing crashes and no one dies. You can experiment and should. You are under no obligations to take anything as settled or sacrosanct.