I use this combination of knots every time I want a very tight rope line like attaching something to the roof of a car or tying down a tarp. It turns a single piece of rope into a simple machine similar to a pulley, increasing your mechanical advantage when tightening the rope.
Step 1: Overview of the Setup
The bowline secures one end of the rope. The butterfly acts as the pulley. Tied in the middle of the rope it allows you to double back and increase your mechanical advantage. The tautline finishes the setup by creating an adjustable knot that you can slide to pull tighter.
Step 2: Secure One End (Bowline)
This may depend on what you are trying to secure, but to secure one of the rope ends I often use a bowline which is a fairly secure knot that won't slip. If your rope is already secure you may not even need a knot for this part.
Step 3: Creating the 'pulley' Knot (Butterfly Knot)
The Butterfly knot is to the key to the mechanical advantage. It is pretty easy to tie and has two nice properties. The loop won't slip and the knot gets tighter as the the two ends are pulled.
You will want to tie this in the rope somewhere between the two points to be secured. It will depend on how much extra rope you have and how much you want to tighten the rope down.
Step 4: Tightening the Rope (Tautline Hitch)
To finish you wrap the rope around the security point (or through a hole if attaching to something like a grommet).
You then thread the end of the rope through the Butterfly knot's loop and back toward the second secure point. You can now tighten the rope and use the mechanical advantage provided by the Butterfly knot.
Make the rope almost as tight as you want it and then begin to tie the Tautline hitch. This hitch as its name implies tightens the line. Once tied you should be able to slide it closer to the second secure point without it slipping.