The midshipman’s hitch is a great little knot which I use all the time. Generally I use the midshipman’s hitch to stop my tarp moving along its ridgeline and also for fastening my guy lines.
The starting point of this post is that you’ve already set up your ridgeline, running through your tarp fixing points.
I leave a piece of paracord about 20cm long tied to the loop on the tarp through which the main line runs. Run this piece of paracord parallel with the main line.
Put a half hitch around the main line.
Add another 3 coils.
Pull the paracord parallel to the main line in the same direction as it was originally going.
Pull a loop through.
Pull on the standing end and the loop tightens. The beauty of this hitch is that it will only travel in one direction, away from the tarp. Move to the other end of the tarp and tie another midshipman’s hitch.
You can then use exactly the samer hitch to fasten your guy lines.