If your mast has double spreaders, you need to make a special sling that would slide up just above the centre of gravity of the mast (as Ted pointed out) AND attached to the solid point of the mast (so it cannot slide higher, e.g. to the goose-neck) - this would be the load bearing attachment when the mast is vertical.
The centre of gravity for the mast is above the lower spreaders and below the upper ones!
There are ways of tying the sling without climbing above the first spreaders, but it is much easier to climb a ladder and tie it there.
Good luck