What fabric do you use for a mockup? I use old sheets. Not ones in your current possession. Go to a thrift store, like a Good Will or Value Village and find some reasonably clean sheets for just a couple dollars. When you get home, WASH THEM. Who knows what mung might still be on them. I got a white fitted for $3.00 USD at Good Will. I bleached it, and once the cycle was done I washed it in regular detergent. Now thats a clean sheet!
To work with a fitted sheet, you have to make it lie flat. So what I do is I find the corners where it is stitched together, and I cut along the seam. Do this for all four corners. Now, locate the elastic around the edges. Some sheets have this all the way around, some only have it on the corners, and some have it along two sides. Cut the elastic off, closely. We don't want to lose too much fabric, but we don't want the elastic causing bunches.
So once you have your sheet spread out, find something to hold down the corners. I used canned goods. Why? They are easily on hand, and you don't have to go out and buy unnecessary crap! With many budgets these days being tight, the less you have to spend overall the better, and canned goods are helpful, even if you use them as weights. =)
So, assuming that you have cut out your patters for front and back, arrange them on the sheet with the grain as best you can. I don't want to mess up my patterns by punching holes in them with pins, so out come more canned goods as weights! Tuna and kippers are good fore more than just eating, they make plentiful - don't forget tasty- and inexpensive weights.
Now, take a pen or marker of some kind and carefully outline as close as you can to the pattern. Use something that won't bleed through onto whatever surface you are working on.
Next, cut the sheet to a more manageable size, and then cut out your patterns. I left approximately 1/2 inch around each piece to have room for letting out during the fitting if necessary. Better to guesstimate on the big side and take in as you go, than to not have enough fabric and be wasteful by losing a whole pattern.
This is only half of your pattern though, so you need to repeat the process for the other half!*