I go into YSSY in a Cessna all the time.
Its a piece of piss, however pretty much everyone will make fun of you from tower to the 747 told to lineup behind you.
edit: I'll be more helpful. Ring up the flow controller in the morning via the phone number in the ERSA and book a timeslot. In the middle of the day YSSY is dead so I've never been rejected (and i'm talking the last 8 or so times i've been in there) so aim for around then. They will ask you some basic info like a/c type etc. The slot time you're given is for LANDING at YSSY, so ensure you get your estimates right to account for that.
Calling inbound I usually do on radar (im going in VFR) 124.55 or 125.8 and I call within 45 DME with the SY ATIS. I usually get put on 135.9 after Radar asks me sometimes up to 4 consecutive times that I'm sure I actually want to land at YSSY.
Inbound procedure is usually just whatever the controller can think up. I've had everything from direct, to track OCTA Victa 1 and report at Botany bay heads, then direct the threshold of 34R.
Once on the deck have a good sized airport diagram with all the taxiways. Know which taxiways are 1 way (B, C etc) and when to turn onto SMC (when west of taxiway S) etc. Other than that, ask to park at Domestic 5 which is opposite Krispy Kreme. Thats gate 9. Security used to let me through there, but no longer, they had their access changed so you need to go through the general gate. Good luck with that.
Either way its lots of fun and I have a many pictures and videos. Quite memorable.