It used to be super-simple, but SunPass is in the throes of switching their computer system to an outside vendor and it's been a nightmare so far. The system itself works great, but the vendor is apparently having great difficulty...and not just in Florida.
The instructions are included in the packaging. Basically you set up your account, link a credit card for automatic replenishment, set a replenishment amount, and enter your car info (make, model, tag, etc.) Then you just drive through the SunPass lanes and skip the toll booths. SunPass tolls are somewhat less than the regular tolls, and if the SunPass reader doesn't work, you get billed by your license plate at the discounted SunPass rates.
If you are just using it for a Florida vacation, I would set the replenishment amount at the minimum $10 so you don't end up with a bunch of money on the account and have to work out a refund.
There are two SunPasses. One is a disposable sticker which costs $5; the other is a portable unit which costs $20. The portable unit could be used with the SunPass app for your phone, but I don't know if that will be the case with the new system. We used the stickers for years without problems.