Anathema is the first mission in Hitman 2 (Gontranno Sanctuary notwithstanding,) but the game's general issues can make it hard to go through. So, in the event you need one, here is a guide!
As the postman[]
If you were paying attention during the opening cutscene, you should have noticed the postman. He is the man walking with flowers by the gate. He will eventually go to take a piss by a tree. Simply enter sneak mode and use the anesthetic to knock him out (he almost certainly will wake up by the end of the mission, so if you want to kill him, it's understandable but completely unnecessary.) If you're playing on a difficulty higher than normal, you will need to drag him behind the gate, so he doesn't get found by guards.
Upon taking his disguise, you will get the message that 'postmen don't carry guns.' In order to bring your guns inside the compound, go north from where you are, and you should find the delivery boy's van. By putting your guns in the nearby crates, they will be brought inside for you! Now walk up to the villa entrance (with the flowers,) and you will be frisked. Once that's over with, step through the gates, and you're in!
When you enter the villa itself, a maid will take the flowers and head through the central door to deliver them. The guard in the room will follow, but only for a short time, so as soon as they leave, head through the door on the left! You might see the maid going up the stairs in the adjacent room, but she doesn't seem to care about you, so go through the doors to kitchen.
Make sure you're sneaking inside the kitchen. By now, at least one of the delivery crates should have made it inside, so pick up your gun(s,) and sneak up the staircase.
Upstairs, there is a door leading to a balcony. This balcony is mostly gated, keyword being mostly. You can sneak along the side of the villa using the little ledge there.
You'll find yourself on a roof. Nearby is the Don's office. Check the map if the Don is inside his office, go in through the long door and wire him. If not, you'll have to wait. Either way, it's a simple kill. Make sure to take the keys off his body!
Head back to the ledge (be careful as depending on your timing, you might get seen on the roof!) Once you're above a staircase, drop down, and go down the stairs. Getting inside the room down here is one of your objectives, and once you do, you'll be free to leave.
On your way back, watch out for a guard that wanders between the garage and the outside kitchen entrance, as he can be a nuisance. Once past him, simply head towards where you started.
As the delivery boy[]
Head towards the delivery van north of where you start. If you're quick and stealthy enough, you can knock out the delivery boy before his first trip. Otherwise, you'll have to wait.
Upon gathering the delivery boy's disguise, head inside the compound. You won't get frisked at this side entrance, so don't bother hiding your guns.
Obviously, we want to get inside the mansion, but the chef inside the kitchen will instantly see through your disguise, so head down the nearby stairs instead.
This is the basement. We can't complete our objective here yet, so make a mental note of it. Up the opposite stairs is the kitchen, which you will need to sneak through (watch out for the wandering guard!)
Make your way up the other set of stairs in the kitchen. Through one of the doors here is balcony, but part of the OSHA-regulated barriers has broken! You can sneak on the nearby ledge, all the way to an opposing roof. Check your map to find the Don, and once he's in his office, you're free to kill him. Exiting is a whole other story, however. Depending on where the Don's la famiglia are, going back the way you came might be a no-no. You may have to kill a guard and take his robes in order to leave, but you can still get the SA rating doing this.
Enter the basement and unlock the room with the Don's key. Now you're free to exit, so just go back to where the mission started.
(Bonus: There is a car key in a tree from the rooftop near the Don's office (Check the gallery on Anathema's main page for specifics.) You can use this car key to exit using the car in the garage, which doubles as an easier way to obtain the R93 Sniper Rifle (Not that it was hard to get.))
As a guard[]
From the start of the mission, head for the door on the side of the compound walls. Hide behind the tree here. A guard will eventually come to piss on the opposite set of trees here. Use your anesthetic to get his disguise.
Inside the door, you can head into the garage and grab the R93 Sniper Rifle here. A good vantage point to snipe the Don from is the starting point of the mission, on the hill.
Upon killing the Don, check where his body fell. If it's still on the balcony, you have to go through the villa to get the keys on his corpse. If it fell down to the grass, congrats! You only have a somewhat-lengthy walk to his body! Either way, use the stairs to the right of your entrance to the mansion to get to the basement, and you're good to exit.