Remember, the cat curve on fabric is not the same as the classic catenary curve formed by a line, cable or chain. The fabric curve is complicated by the fact that some of the pull is on the bias of the fabric - the angle at which the pull is not parallel to the weave of the fabric. Therefore, the curve for a ridge line does not follow the catenary formula(don't ask). Fabric distorts more than a line and needs a different catenary shape.
There are two good ways to establish a catenary curve. The easiest is to get a long baton - either a piece of wooden molding or, better yet, a bar of aluminum (something like the 3/16" X 1" stock available in most hardware stores). The aluminum bar is a good straight edge when laid flat and, turned up on its side, will bend uniformly when used to establish a curve. Lay out the cloth flat and straight and tape it down. Then use weights such as bricks at the ends of the ridge line to hold the bar in place. Offset the center however much your design requires using another brick to keep the baton in place. I use at least 1/2 inch offset per foot of ridge line. That is an arbitrary number that puts a 5 inch curve on a 10 foot ridge line. More will work, but reduces the tarps coverage area and headroom. Less may not pull it taut. If all you want to do is make your sewing mistakes less visible, reduce the offset to 1/4" per foot. That works for me. It does not give a firm catenary. I use it on "square" tarps - those without catenary.
The other method is trickier, but gives an exact catenary for the fabric you are using. Lay out one panel of the fabric. Anchor each corner firmly especially one ends of what will become the ridge line. Then stretch the ridge firmly and uniformly from the each end by moving the anchors. This will pull the fabric out of shape and will produce the very wrinkles the catenary cut is designed to remove. Anchor the stretched fabric and use chalk or Magic Marker to dot along the part of the panel that is mostly not wrinkled. This is usually the between the last large wrinkle and a smaller, half-hearted wrinkle before the fabric levels out. These dots are unlikely to form a neat line, but they will serve as a general guide for laying out the baton or bar as discussed earlier. This curve will not look exactly like the other curve, and will need two center anchors. No matter, it will still pull the tarp taut. Mark and cut one panel, then use it as a guide to mark and cut the other one.
Personally, I prefer to lay out a simple curve as in the first description. It works, it is easy, and I have messed up with the second method. The only virtue of the second method is when using unfamiliar fabric that has more stretch than normal, such as PU coated ripstop nylon from an unknown source. That stuff can really sag and needs a lot of catenary. The stretch method will show you how much.