Loose or sagging plastic gutters will not drain properly, and this may cause their contents to overflow. As a temporary measure, hammer in a wooden wedge between each bracket and the gutter to hold each section in place until you can replace it.
When you need to duplicate multiple measurements on a larger project, it can be helpful to use a length of 1×2 or similar stock as a story pole. Cut or select a piece that is longer than the longest cut required. Carefully mark the height, width, depth and length of various parts onto the 1×2. The story pole will speed up marking for cuts, and matching parts will be cut the same.