They started shooting at the top of the staircase by the west entrance where Rachel Scott was sitting and eating lunch. After they killed her, they started shooting down the staircase. After they shot the kids on the stairs, Dylan went and peeked inside the Cafeteria but he then went back outside, ran back up the staircase and he and Eric entered the school through the west entrance, right where they shot and killed Rachel.
Once they got inside, they ran around the hallways a little bit and then went into the Library. After the Library, they went down to the Cafeteria. This is when you can see Eric on the CCTV footage at the bottom of the Cafeteria staircase shooting at the propane tanks.
They messed around in the Cafeteria for a little bit and then walked around the school some more shooting at lockers and lighting bombs. Then they went to the Cafeteria a second time. This is where the infamous "11:57" photo came from - their second time in the Cafeteria.
They fooled around in the Cafeteria and at 12 on the dot, you can see them on the CCTV footage walking back up the staircase towards the Library. They went into the Library and shot at cops through the windows for a few minutes and around 12:08 Eric killed himself and Dylan was shortly after.
"Someday, you will ache like I ache".