I realize this isn't rocket science but figured it might somebody so here's how I marked up my 7 gallon Fermonster.
First I ensured it was completely dry, hooked up the spigot, and calibrated my water to around 68 degrees.
Next I started the process of measuring out 2 quarts of water, pouring them into the fermonster, marking just below the water line with 4 inches of masking tape. I mad sure both ends of the tape were equidistant from the water line so the lines would be as straight as possible. This worked well until gallon 7 and above as they are on angles. For those I just tried to get the middle as straight as possible. Not too concerned with the accuracy of those anyway.
I found I could get into a good rhythm of refilling the pitcher while waiting for the addition to the Fermonster to stabilize and likewise I could mark the Fermonster while I was waiting for the pitcher water to stabilize.
Next simply use a permanent marker and your still hand of choice to draw lines above each piece of masking tape. I choose to do 1 inch lines for gallon markers and .5 inch lines for the 1/2 gallon markings.
FWIW, I measured the capacity of my 7 gallon Fermonster at slightly above 7 11/16ths gallons. That is with the spigot which takes up some volume. Also the 5, 6, and 7 gallon markers were all off by quite a bit as pictured below (look for the dimple which is about 1/4 inch higher than my 5 gallon mark. Note the cool bubble lines that naturally occurred during this process at each 1/2 gallon marking...too bad I couldn't use those to somehow chalkline the fermonster.
Also if you make a mistake with the permanent marker, you can use dry erase markers to draw over the permanent marker. It acts like an eraser and then you can wipe both off easily.
I'm not sure why the pics I upload always show up rotated incorrectly...I am positive they are the right orientation when I upload them. Any advice?