Don't forget that the cherry bag is going to get into the fermenter so you need to leave a space for that.
Remove leaves and stems.
Give a good wash to the cherries (just water).
Put the cherries in a steeping bag and while your wort cooling put the bag inside (enter it around 65 C for 15 minutes (50C))
Put the bag out into sterilize vessel and wait til your wort cool down.
Mash and squeeze the cherries in your vessel.
Add the bag (with the juice) into the cooled wort again for fermentation.
Don't forget to get OG after you added your cherries because they contain a lot of sugars (still your actual OG is bigger by few 0.00X because there is still sugars inside the cherry that the wort not contain)