The old advice to measure twice and cut once is good advice for sewers. Sewing involves having end product and that involves planning.
This photo shows the results of not planning carefully. Knit fabrics need a smaller needle size. I didn't have one and thought I could make this top with a size 14 needle. I went ahead and finished the whole thing, wasting the money I spent on fabric and worse, a lot of time. It was, however, a learning experience and a reminder to take the time to plan: get all your materials together including the right tools. Get your scissors sharpened. Spend a couple of bucks on new needles and take the time to read up on the fabrics you are using. There are several websites that are have sewing help. Then you won't end up in yoga class coming undone at the seams!