How do you accurately mark fabric without a pattern?
There is always the option of making a pattern - i.e. an appropriately sized piece of paper. Then you can use it as you are accustomed to.
For rectangular patterns, remember that when you fold an edge onto itself, the resulting crease line will be perpendicular to the edge. This helps a lot when constructing a rectangle.
So if your fabric is sturdy and not prone to distort, arm yourself with an iron and start ironing, folding, and ironing, etc., always measuring your distances. If however your fabric won’t necessarily hold its shape, I recommend the homemade paper pattern route.