One of the parts that hurts my head most is making the pattern. haha it requires thinking. :/ I start by taking pictures of all the shirts, and putting them into a program so I can move them around and see how they look. I love Adobe Fireworks for this part. You can probably do it with something like Word though if you don't have Fireworks or something similar. Something I have found super helpful is putting all the shirts I want in a folder and after making the icons extra large hitting the print screen button. Then I paste them into my editor program and crop and move them from there.
Print Screened Image
After I have all the pix cropped and put onto my blank space I go and measure all the designs. Sometimes I will print out a picture with the shirt designs on it so I can write the measurements directly on top. I use a 1/2" seam allowance so I add an inch to the length and the width. This is where my head starts to hurt. Once I have all my measurements then I go back into Fireworks (and this is what I love about that program) and size each picture to scale. I use 10 pixels to an inch. So if my width is 13 inches I make it 130 pixels and it works pretty good for me.
After they are to scale I start moving the pictures around to where they look good/ where they will fit size wise. Then take your largest width and make it the same for all in that column and take your largest height and make it the same for all those in the row. If you have a square with several designs in it don't forget to adjust for the seam allowance. (1" for every seam.) I don't know why but I put width first and then length when writing them on my pattern. Make sure you do it the same way every time. You can do it opposite this, but don't switch back and forth when writing it down or it will get super confusing.
I like to make my background to scale as well so I know how much room I have to work with. I add an inch or 10 pixels for each seam. This allows me to know how much space I will have in between each shirt. With the white up above I put it how I wanted it to look in the end instead of with the seam allowance and it messed me up for my end result measurements. So make sure to be consistent one way or the other. Don't mix and match there either or it will alter the end result unless you are one of those people who can keep it all straight in your head. Then more power to you!
One more tip when buying fabric I like to buy enough for the width of my quilt. So if I'm making it a twin size I'll buy enough for 60" width that way I can just have one solid piece all the way across. (Unless I'm doing an ombre quilt then I go the cheaper route and piece it.)
I can't remember where I got these numbers, but they have worked pretty well for me so I'll pass them onto you.
King 96"x100" 44-52 designs
Queen 96"x82" 36-44 designs
Full 90"x76" 30-36 designs
XLong Twin 96"x60" 26-30 designs
Twin 90"x60" 22-26 designs
Lg Throw 70"x60" 16-22 designs
Sm Throw 60"x50" 10-16 designs
If you want a particular size, but have only a few designs you can add some decorative elements. I made my grandmother one out of her old shirts and she only had 6 shirts, so I was able to play around a little bit. It's actually one of my favorites.