Part of the joy of home sewing is to get clothes that fit you perfectly, this is where toiling is so important. Toiling is basically the practise run for your final garment. You make it up in a similar weight, inexpensive, fabric to give you an idea of how the finished item will look and how it will fit you. Usually made in calico. It also gives you a practise with the construction of the garment so when you sew the final one you have a better idea of how it all fits together.
We can't stress enough the importance of toiling here at Trend Patterns. Mostly because we belong to the BBTT crew (big bum, tiny tits) which means a lot of the things we make need adjusting to fit us, because unfortunately, we're perfect in every way except proportionally. We usually need to let out a bit at the hip or take in a bit at the bust to get something to fit perfectly, so without doing a toile first we would be left with something that doesn't fit right at all. Thats why we would advise toiling all new patterns before going ahead and making them in final fabrics.
When you're sewing a toile it doesn't need to be finished perfectly, you don't need to finish the hems or overlock the seams or anything like that, you just need the basic shell of the garment so you can see how it fits you. Definitely don't be precious about your toiles as the likelihood is you will need to pin bits out or cut bits up to get it to fit correctly.
When you fit your toile you ideally need an assistant as its near impossible for you to see how the back looks and even harder to pin it.
You need to look out for areas that either look or feel too tight or too loose. If its too loose its easy to pin out the excess - its best to pin along where you have seams so you can easily take out the extra invisibly. If its too tight you'll need to cut along the seams and pin in an extra piece of fabric underneath until it feels comfortable.
We have a more detail post about fitting and adjusting your pattern accordingly so keep an eye out for that.
If your toiles look great you could always wear them anyway in a Margiela sort of way.