When I make costumes, and I'll say I'm doing the mock up, some people give me rather blank stares and have absolutely no idea what I am talking about.
Basically, a mock up is a test of whatever pattern you are using to make sure it fits before you cut into your 'final' fabric. Some people swear by muslin, but I prefer using an old sheet or whatever cotton fabric I know I won't want. You piece together your sheet after cutting out the pieces, and then try it on and tweak it to make it fit. Here are some pictures I took of myself when I was trying to determine what to change. You may notice how in the pictures, I sewed up the back and pinned in the front, even though in the end it would be a back closing dress:
Thankfully the first costume I made in which I thought I knew what I was doing but really had no clue, it turned out to fit me pretty well. Sadly, the I made four costumes before I even heard what a mock up was. The first three were loose enough that it didn't matter, but I tried to make a tight fitting dress and it was much much too tight, so I only wore it once.
That same tight dress I came back to and made a mock up in the same size as before, and I was surprised to find it fit perfectly. What I had done wrong was I made it completely, and then tried it on. It came down too low, so it seemed like a good idea to simple take it in in the shoulders. That was the worst mistake ever. The finished garment the armholes were so tight from where I took it in, and the front bunched horribly because the bust seams, which curved with me and would have fit fine, had been pulled up too much by the adjustment and the whole thing wrinkled up. I wish I had some better before and after pictures.
Here are some tips I have found very useful when making a mock up:
1. When sewing the pieces together, use the largest stitch available. That way, when you tweak it, you can easily rip all the pieces apart and use that as your pattern instead of the tissue and then have to remark the changes from your mock up to the actual fabric.
2. Use a different color thread from the mock up fabric you are using; seeing the seams is easier when there is a contrast.
3. Before putting on the mock up, make sure you wear whatever undergarments you will be wearing when it is finished. Also put on a tight-fitting tank top.
4. When trying on the mock up, put it on inside out. You can then pull in seams where it is needed just by pinching them to whatever size you need, then pinning them (be careful not to poke yourself!).
5. When I put it on, I have found that I like to pin the mock up to my tank top at the side seams and the center front. If I'm working with darts or gathers, then I can do it evenly.
6. If the pattern you are using has darts, don't cut into them for the mock up! Instead, mark where they will be on the mock up, then pin them in. You don't really need to stitch them in on the mock up.
7. On one of the back closing dresses that I made, I found it was helpful to baste up the back exactly how it would be when I was done, then keep the front open (there was a center front seam). Then I pinned the front with a half inch seam, like how it would be done when it was finished.
8. Don't be afraid to cut into that mock up! I have found that lots of patterns make the arm hole too small, so on the mock up I'll cut into the back part of the armhole quite a bit.
9. Also don't be afraid to mark your mock up! On the last pattern I did, I got tired of making notes, so I took a marker and wrote all my changes on the mock up, then took it off and took it in in some places.
10. Not all the pieces from your pattern need to be turned into a mock up. I usually leave out the sleeves, and making an entire skirt for a costume usually takes more time than it is worth. I only add the sleeves if I think there is a chance they won't work, such as if the sleeves are particularly tight, or in the last case, the mock up sleeves were way too small.
11. If your first mock up is a complete flunk, such as it is too small or it is way too large, cut another one in a different size! I have one friend that drafts her own patterns, and does many, many mock ups on just one part of the pattern. She likes to make sure it is perfect; then the piecing together process goes much faster. On the other hand, I have a friend that always does only one. She slashes into that thing, and is really more of a measure once cut twice person. In her case, it takes more time for her to complete the garment, because she needs to make more changes to the actual finished product.
12. One of my friends really likes to really only focus on one side of the pattern. It really works well; she makes both sides of the mock up, but only makes changes to one side. In this way, it saves time and you don't have to get both sides perfectly the same. If you try this, make sure you pin your mock up to a tank top to make sure it doesn't slip around and the front stays in the front.