Start at the inner edge of the window frame and measure to the opposite inner edge to get the window's width. You'll use the width to purchase to the right size tension rod for your curtains. Tension rods have a spring mechanism inside of them that creates tension when the ends of the rod are pressed against a window frame. If the rod is too short, there won't be enough tension, and if the rod is too long, there will be too much tension. Tension rods generally list a window size range on the packaging, so look for one that will work based on the width of your window.  You can find tension rods online or at your local hardware store. If you have particularly heavy curtains, you may want to try adhesive strips instead of a tension rod. A tension rod is best for small, lightweight curtains. The right way to adjust the length of your tension rod will depend on the model you're using, but generally, you'll pull or twist on the rod. You want the tension rod to be slightly longer than the width of your window so there's enough tension to hold it in place. If you're not sure how to adjust the length of your tension rod, refer to the instructions that came with it. Use a pencil to draw a circle around each end of the tension rod where it pushes into the frame. Use a level to make sure the rod is even before you make your marks. Slide the hoops or fasteners on your curtains onto the rod. Make sure all of the panels you want to use are on and facing the right direction since you'll need to take the tension rod down if you want to adjust them. If your curtains have clips instead of hoops, you don't need to take the tension rod down to hang them up. Insert the tension rod into the frame so the ends are aligned with the marks you made earlier. If the rod doesn't feel secure, you may need to take it down and adjust it so it's longer and there's more tension.
Measure from one side of the window to the other. Buy a tension rod that will fit inside your window. Adjust the tension rod so it's slightly longer than the width of your window. Insert the rod into the window frame and mark where each end is. Take down the rod and hang your curtains on it. Hang up the tension rod and curtains.