You cannot. Gensokyo doesn't seem to occupy any space in the outside world, but the Hakurei Border does envelop a certain space in the outside world. That space is what connects the outside world to Gensokyo.
-Yukari can enter and leave Gensokyo on what seems to be her whim. Yukari spirits away humans sometimes for reasons unknown.
-Yukari has opened the border to the Lunar Capital on a few occasions, which I suppose also counts.
-People can wander into Gensokyo by staying around the outside's "half" of the border (which also has a small shrine on its edge); despite how strong the boundary is, it is not perfect. The area has magic that makes people wander away from it, but random objects and people sometimes find their way in. Reimu has the duty of either sending these people back or sending them to the human village if they wish to stay.
-There have been presumed to be other entrances besides the shrine that were ripped open by presumably strong youkai. Therefore the shrine in the outside world is probably also not the only point of entry.
-Chiyuri and Yumemi made their way in by space-time travel of sorts.
-Maribel enters Gensokyo through her dreams.
-The Scarlet Devil Mansion was transported to Gensokyo through unknown means.
-Reisen made it into Gensokyo through unknown means.
-Toyohime and Yorihime travel to Gensokyo through unknown means, but it is presumed that Toyohime's ability to connect Earth and Sea is involved.
-The Moriya Shrine was transported to Gensokyo through unknown means but was the doing of Kanako and/or Suwako.
-The Ruined Western Mansion (home to the Prismrivers) was transported to Gensokyo by Layla using magic of sorts.
-Mamizou just paraded in somehow. It's possible Nue left Gensokyo to retrieve Mamizou, so getting into Gensokyo could have been her doing as well.
