Your suit bottoms should have little loops in them. You run the strings of the top thru them and tie them down. Personally I DON"T recommend tying them in a knot cuz if you have to use the restroom or something while you are strapped in (e.g. 2 min before you go on stage after running your diuretics for 10 days....i.e. you GOTTA GO) cuz you will never get the knot out. I suggest instead to put safety pins on the ends of each string. That also helps in running the strings thru the loops. Then pin them down to the suit. YOu will probably want to cut the strings to length so you don't have excess sticking out of your suit on stage. But also make sure you pin in tightly to the bottoms so the pin isn't exposed where the string pulls. Make sure its a secure connection at the pin-string and pin-bottoms points so no where does the material slip or the string unravel. If your string is really thin, then you could probably put a knot in it and pin thru that.
There's another thing -- bring a pair of scissors to the show in case you need to clip down the strings. But don't clip them too far down or else you are just SOL.