Saber saw is almost useless for that thick.
Do you have a table saw or hand saw, either will work although that size will be a bit heavy to handle on a table saw.
Get a 7-1/4" hand saw and buy what is called a triple chip blade; this will be marked for cutting non-ferrous metals.
Support your aluminum on strips of wood on the floor so there is a gap underneath for the blade to go through.
You will be able to freehand cut but sometimes it is easier to clamp on a guide; just a length of wood will do.
Mark your cutline and paint some ATF fluid along as a cutting fluid/lubricant; it works really well.
Get yourself nicely positioned so you can guide the saw with both hands. Full face shield on and hearing protection; hot chips are going to fly everywhere and it is going to be noisy.
Start into your cut. Do not push to fast, just a nice steady pressure and not much pressure with a new blade. You will find there is a rate at which you can push where the blade seems to float through the metal cutting very nicely, if you try going faster it fights back.
Sometimes the chips do not clear out nicely and jam in the saw teeth and you will hear the saw labouring. Stop pushing but keep holding the saw down against the plate as you release the switch and let the blade stop completely. Do not try to lift the saw yet, unplug it. Now lift the saw out of the cut and see if there are chips packed in the blade. You can normally push them out or tap them out with a bit of wood.
Slobber on more lubricant and restart your cut from the beginning. When you have plenty of experience you can maybe restart where you left off but doing this can lead to a powerful kick back if not done correctly.
I have not mentioned clamping the material down because your piece will be so heavy and you will probably be kneeling on it on the floor. When you have smaller pieces then you must clamp them down firmly and make sure your clamps are not going to be in the way.