Note: This knot works better in smaller diameter cords, but its easier to demonstrate with larger ones so that is what is in the pictures.
To start with you go over the top of the rope and then do 2-3 coils back towards the what will be the closed loop before bring it back over where you started.
Then you go underneath the main line,
Then you coil again 2-3 coils in the direction of the closed loop making the coils on that side match the other.
After it is all dressed nicely it should look like this,
The advantage of this knot for adjusting tie down strings on tarps and tents is that you can slide the knot towards the tent to get it tight and then the prusik will hold it in place. It's fairly easy to untie because only one end of the prusik has tension on it, you can just undo the unweighted side and then the other will come right off.
Do note that it's the coils towards the closed loop that will take the weight, sometimes its worth putting 4-6 coils on that side and only 2 on the unweighted side.