Step 1: Estimate the total length
Measure your item and round up or down to the nearest length that you can easily divide by 2. My window is about 25 13/16" wide. I'll round that to 26", which gives me 13" when divided by 2.
Step 2: Measure from both ends
Take the number you got from step 1 and measure that far from one end. Make a mark. Repeat from the other end. You should now have two marks that are pretty close together.