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The building entrance is close to a roundabout with a sculpture, and is clearly signed. Coming from Bolivia, it's on the righthand side.
© OpenStreetMap contributors
The area is not well signed. The border post is a long building, and the immigration post is reached through the side closer to the lake shore. Officers might occasionally ask for a copy of the entrance stamps - fear not, there's a photocopy service exactly across the street from the immigration room, copies for Bs. 0.50 each side.
Muelle
If you really have nothing else to kill time on, contemplating life gazing at Lake Titicaca is the way to go. Keeping your sight far away helps to ignore the trash near the shore, but the landscape is gorgeous. It can be windy and chilly there, even under the sun - bring a coat.