Start by reading over your pattern if using one. Usually there is what looks like a sideways “V” (>) jetting out on the the side. It will “dart” out to a point so to speak. There will be a line down the center of the “V” to show the center of the dart. (Illustrations to come) This is where you will fold it before sewing.
The next thing to do is to mark your dart, transferring it from the pattern to the fabric you are using. I prefer chalk markers for this but you can use the sewing marker of your choice.
When you are ready to sew your dart, fold your fabric, right (decorative) sides together, using the markings you made, folding it down the center line so that it becomes half of the “V”.
I find it is easier to start sewing at the outside edge and sew inward towards the end point of the dart marking. Use caution when back stitching. It can be difficult at the end point to make it look smooth.
Sew along the line you drew. Use a back stitch on the outer edge and if fabric is lightweight, just run off at the point and tie a knot manually to secure the thread. Repeat for any other darts you need to accomplish and you are done!