Do you mean jacks or jackstands? If you mean jacks, you would not be safe underneath a carrier using those for suport.
If you are talking about jackstands, and using a floor jack, I would reccomend the following method:
Jack up the rear at the differential using a floor jack. Support the rear with two jackstands by placing them on the axle tubes.
Jack up the front of the hull using the floor jack, until the hull is level. Place jackstands under the flat portion of the hull.
When removing the stands, do so in the reverse order. Jack stands are made to be supporting a round surface and in the middle of the stand, so you do not want the weight of the carrier on just one finger of the stand. That is why I reccomend the use of the axle tube on the rear, first on and last off. Otherwise you may learn just how fast 4 tons of metal can shoot an improperly place jackstand.
You may want to place a short piece of 2X4 between the hull and the jackstand, both for the benefit of the jackstand (gets the load to the center of the stand instead of on the edges) and for the benefit of those thinner Ontario hulls.
The manual shows the crosstube as a jacking point, and while this may be fine to change a roadwheel, without blocks under the vehicle, is not safe to work under.
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