By default (until you reach higher mastery level) this mission can be started only at the fountain in the eastern corner of the map (M3,1). You don't need to obtain a disguise or start sneaking up immediately - the guests that arrived at the palace can freely move through some parts of the building. In addition to the starting locations (eastern gardens, southern parking for personnel), it applies to the biggest rooms on floor 1 of the palace - the main hall (M3,19), banquet hall (M3,20) and the catwalk (M3,21). The guests can also go to the northern terrace that neighbors the banquet room and to the nearby part of the gardens.
In order to be able to move freely through the floor 1 of the palace you should obtain some new clothes. It is best to obtain a staff or stage crew disguise. Palace Staff Disguise can be obtained for example by reaching the small building located north from the starting location (M3,3). You can do that by sneaking up through the gardens and jumping over the fence. Inside the building there is a staff member. You can stun him and hide his body in the nearby container. If you don't want to attack anyone of the staff, you can go to one of the trailers in the southern part of the map (M3,4). You can find an unused disguise there. After wearing it you can freely travel through floors 0 and 1 (except for the gazebo in the western part of the gardens (M3,2) - it is related to one of the assassination opportunities).
The easiest place to obtain the Stage Crew Disguise is where the technical crew of the show is stationed (M3,14) in the southern part of the map. There you can locate the generator and after sabotaging it wait for the technical worker to come. Then stun him and hide his body behind the wall. You can also obtain this disguise by knocking down the person traveling through the southeastern rooms on floor 1 of the palace (M3,22). The best time to attack is when the person is the room with glass displays or in the nearby toilet. Once you put the disguise on, you can travel through floors 0, 1 and 2. The only exception will be the apartment on floor 2 (M3,7) to which the stage crew has no access.
Floor 0 of the palace (the basement) can be explored with no trouble if you obtained one of the disguises suggested above. If you don't want to use any disguise, then ignore all the main basement entrances - there is a great chance for you to find someone there who would raise an alarm. Instead of that, use the small staircase in the central part of the building (M3,18) - it isn't guarded by anyone. While in the basement, you can find another unused staff disguise. It is located in the locker-room (M3,6). You should also remember the location of the monitoring room (M3,17). If later in the game you are recorded by cameras, then you can go back here and destroy the recordings. Ignoring this step won't mean failing the mission, but it will have a negative impact on your score.
The easiest way to reach floor 2 of the palace is by using the stage crew or CICADA Bodyguard Disguise. The latter can be obtained in the northwestern part of the gardens (M3,8). Two bodyguards are walking there. Once one of them is alone, simply attack him and quickly move his body to one of the nearby containers. Another advantage of the bodyguard disguise is that you can use it to travel freely through the highest, third floor of the palace. After putting proper clothes on, you can use any of the main stairs leading to floor 2 - for example the ones in the main hall (M3,19) or in the back of the building.
If you want to reach floor 2 of the building without using any disguises, then you should do it by obtaining the IAGO Invitation. It can be found in one of the toilets in the northern part of the building, on floor 1 (M3,8). Once you take it, go to the northern terrace, enter the northern gardens and turn to the west. Reach the guards protecting the entrance to the back of the palace (M3,6). Show the invitation and walk through. You will have to show the invitation to the guards for the second time once you reach the stairs leading to floor 2.
In order to get to floor 2 of the building you don't have to use any of the "main" stairs. Behind the catwalk where the event takes place there is an additional staircase (M3,26). You can encounter a stage crew member there, he will be alarmed if Agent 47 won't wear proper disguise. An even more interesting idea is to climb on the gutters on the walls of the palace (mostly at its back). Additional advantage of that method is the fact that the gutters can allow Agent 47 to reach even floor 3. Depending on which gutter you choose, you can reach a higher balcony immediately or first climb a ledge. In the second case Agent 47 will be able to move to the left or to the right in order to reach a specific window.
The third, highest floor of the palace (the auction organized by Dalia Margolis) can be reached by selecting the "official" stairs (M3,30). In that case you must have the previously mentioned auction invitation or an adequate disguise. If you choose latter, then it is best to get rid of the sheikh (M3,7) or Helmut Kruger (M3,12) (details about how to do it can be found on further pages of this walkthrough). If you don't want to waste time, then you can go to the western room on floor 2 (M3,27). The auction staff member can be found there - you can easily stun him and get Auction Staff Disguise from him.
When trying to use stairs leading to floor 3, Agent 47 will be searched. You can't have any guns or explosives in your inventory or you will alarm the bodyguards (having the garrote is allowed). In theory you can drop these items in any point of the palace, but if you don't want them to be found you should look for one of the special containers in which you can temporarily stash them. What's important, if you taken the identity of sheikh or Kruger, you won't be searched as they are considered being VIPs.
Luckily, you can get to the third floor in few other ways as well. One of the best options is to use the previously mentioned gutters. You can also go to the eastern balcony on floor 2 (M3,29) and find the ladder that leads to the attic on floor 3. The attic is a restricted zone unavailable for guests and even for VIPs. Many bodyguards can be found there. Luckily, you can bypass them (you should do that even when wearing a bodyguard suit - some enemies can still recognize Agent 47).