Ok, first you want to take a piece of fabric that is about 4" wider than you need & in terms of width & for length I recommend using the full yard so you have enough fabric for making the folds & hanging it as low as you want.
For my powder room, I wanted the romans to be inside mounted, the inside of my window is 21" so I cut the fabric to 25" wide. This gave me 2" to fold my fabrics for the border. I had to pay attention to that each shade looked exactly alike with the scallop pattern so after I made the first one I kept referencing it to make sure my new folds were in line with what I did initially (so the bottoms of the folds hit at the bases of the scallops and the sides lined up with the sides of the scallops). If you are nervous I would start with a solid fabric.
So below, is all my sides and bottom folded to meet my pattern needs & width needs, then you'll measure to make sure the width is what you need (again for me 21") at this point the length doesn't matter.
Next you are going to iron down the sides and bottoms so they are crisp.
Then hot glue the bottom flap, then the sides. Next you are going to take a piece of fabric cut to the entire size of the back of your fabric to serve as a liner. This will make the folds look more full & block out more light, making it look beautiful and not like a cheap faux shade :) I was being super thrifty and used an old sheet to line mine so you'll see it's lined with two separate pieces. To avoid headache I would recommend using a continuous piece of fabric for your lining.
Then you hot glue that lining just inside your hems and the top of the fabric. (Again you'll have one large piece here as oppose to my partial lining.