What you want are called line levels, they hang on a string.
These are what masons use as they work. Last ones I bought were about 2$ each.
Another approach is to flood the excavation and level things until the whole surface is evenly submerged .

Run several strings 4' or so apart in the direction you want the patio to slope. Set the stones level to the string, then, use a 2'x4' or metal T-square to level the stones next to them. No tricks....just alot of hard, tedious work. I'm in the middle of our my own patio project...my husband and I can work 4-hour stretches on it and are amazed that we've only set 5-6 stones, if we are lucky, in that time.

If the landscaping company has already put down a base and compacted it, did they not level and slope the base properly?

You can't use the top layer of sand or stone dust to create a drainage slope. Is this what you are trying to do? Or is that done already?

I am having the same problem. I had to lay out the stones tonight to see what I had to work with and ended up with a rectangle shape that is coming out nicely but now I don't know what I am supposed to do. I guess mark it then dig around the outline then take the flagstones off, dig deeper on the higher ground than the lower ground, pour sand in and start the layout process all over again. I plan to grow moss between the cracks. I even bought some Irish Moss at the Home Depot and it's gorgeous. Any suggestions on leveling besides what's on here?

Feel free to email me personally HIWANIUK@AOL.COM