How to flagstone on grass??

First you need to remove the grass😩. Then you smoothe the soil, wet it down to compact it. Spread and level several inches of sand. Then place stones on sand being sure to level each stone ( scrunch down in sand or add more sand to level).Leave room in between stones for a grout line. Spread dry quickcrete over stones. Sweep into areas between stones to create grout lines and secure the stones in place. Once all quickcrete is off the stones grab your garden hose and gently spray the surface to soak down the quickcrete. Allow to cure at least 24 hours. Enjoy!

i wouldn't try putting down flagstone until you either use a sod cutter to rid yourself of the grass or do a kill of all vegetation. Flagstone needs a sturdy base and usually you have to lay down a base of sand and tamp down to make it compacted and level. I'm not sure how involved you want your project to be or how large but those are my suggestions from past experience. Now I do know some of the big box stores have a square fake flakestone that you could use on top of grass to make a small area. Unsure what the cost of these are but they are about 1' sq with a hollowed out botton which I guess you could just lay on top of your area

When we installed our front walkway, we used an underlayment product called HPB (high performance bedding). It replaces the sand. Instead of concrete, you could use polymeric sand; it sets up like concrete. You can check out our front walkway DIY for some tips: https://www.hometalk.com/diy/outdoor/walkways-paths/front-paver-walkway-makeover-38236306

Put down plastic or a tarp for a couple of weeks to kill the grass A little . If you wantbthem level with the rest of the yard you will have to dig out the depth of the flagstone plus an inch. When that is done lay weedblock fabric down, then an inch or sand to lay your blocks atop. You can alter the sand depth for each block to level them. Use a rubber mallet to help you set the blocks. When they are level, pour sand over the top and sweep it into all the joints. If you are laying above grass level then same idea only do not dig out and maybe edge with lawn edging or landscape ties laid side on. I have done this several times when we flipped houses found the smallfoot square blocks easiest and you can checkerboard with red and grey.

Wow, thank you all