Use rope. Ours was cotton-ish, like clothesline rope. Not nylon marine-type rope.
Get at least two people to help... Each of the helpers pulls their end as tight as they can and holds it just to the inside of a leg. You take both ends and tie in a square knot centered between the 2 legs. See below for my attempt at a "sketch."
|| = the leg of the pool
X = helper
___ = rope
+ = you @ the knot
___||_X__+__X_||___
On our pool the rope goes thru guides in between the legs, like the white belt does. Not sure if that matters. I can't tell from your photo if your pool has those. If so, then for whatever reason, your rope was missing, but there should've been one. You could try calling intex....
Another reply mentioned the legs being "properly positioned." This is really, really important, :!: and the installation instructions with the pool are not clear about how important it it to get the legs set perfectly. This means checking all the way around the pool multiple times as it is filling and using a carpenter's level to adjust and re-adjust the legs to vertical. We had to check ours every 1\2 hour, maybe more. This is also easier if more than one person is helping. Once there is about a foot to foot-and-a-half of water you won't be able to move the legs, because the pool will be too heavy. That's why you have to check so often.
After the pool is at least 1/2 full and the legs are set.... Then tie the rope. (We waited till it was all the way filled.)
Oh, and the bottom does bulge out... And the pool frame does flex and move at the joints. It's the way it's designed. The weight of the water is "hung" from the horizontal supports and creates downward pressure which in turn fixes the legs in place. The rope keeps them even, and prevents the pool from "walking" which could happen from the movement of sloshing water creating more force on some legs than others, and the inherent flexing of the joints allowing the legs to give ever so slightly. That's why it's vital that it be set up on a meticulously level area... If it isn't completely level, it will move.
Good Luck.
