Slings should be rigged in a manner that provides proper load control. It is dangerous to use only one sling to lift a load which tends to shift and slide out.
(One sling is depicted for illustrative purposes only).

Ensure that lifting devices are directly over the center of gravity. If this is difficult to determine, it must be discovered by cautious experimentation or calculation. Raise the load carefully. If the load is not level, lower and correct the position of the slings until the balance point is achieved and the load does not tilt.