Although a tenon and dovetail saw have slight differences, they are held in the same way.
Grip the handle of the saw in your dominant hand and extend your index finger along the handle so that it points towards the end of the blade.
Keep your finger extended when working to help you guide the saw.
You can place your other hand on the wood, to steady it (as long you keep it well away from the blade), or you can place it on the spine of the saw to help guide the saw as you cut.
You should try to position yourself directly behind the material being cut.