Xbox game pass ultimate

Hi
Have recently purchased for the kids an Xbox s and want to get Xbox game pass ultimate setup for them, but to be honest, don't know anything about it.

Would somebody be able to provide me with an idiots guide to it please, specifically how to get it long term as cheap as possible (I've previously seen folk talk about stacking multiple years worth?).

I'd be a brand new customer who has redeemed any offer previously. Thanks in advance for the time and patience...