Shimano Hollowtech bottom brackets need a special ring spanner that fits into splines and allows each side to be screwed in and out.
Clean the threads on the frame and grease them with anti-seize assembly grease.
Left To Right
The unit is marked 'L' and 'R'. The right element contains the axel sleeve.
Carefully introduce the right side of the Shimano Hollowtech bottom bracket into the threads, screwing anti-clockwise. Finger power is enough. Too much force may damage the threads.
Introduce the left element into the other side and screw it in carefully with your fingers.
Use the ring spanner to screw the right side of the Shimano Hollowtech bottom bracket tight
Use the ring spanner to tighten the left side.
Remount the cranks.