If unsure a Haynes manual will be a good buy.
Fairly straight forward, pads and disks I'd say on the back? Just jack up, put an axle stand under car before you start. Or at least something solid the jack is not to be trusted.
From memory: Loosen the bottom retaining bolt and this will allow the caliper to hinge upwards on the top bolt. Take pads out and push piston back in fully. Put the new pads into the caliper and replace bottom caliper bolt. Pump brakes to ensure no clearance between pads and disk.
The wheel should turn freely.
You may have one of the first A3's with non ABS brakes they might have had shoes on the back. In general shoes are harder to fit than pads.
Let us know where you are getting into trouble.