Measure the interior width of your cabinet and the width of the shelf you want to install. Confirm that the shelf will actually fit, given the dimensions of your cabinet. If not, you can shave off excess width using a planer or skill saw.
Remove all shelves, shelf brackets and other material from the cabinet.
Slide the shelves inside the cabinet, tilted vertically so they are perpendicular to the direction they will lay when installed. Your top shelf should be on the left, your right shelf on the bottom.
Lean the stacked shelves to the right, until they lie on top of one another at the bottom of the cabinet.
Lift the top shelf and maneuver it until it's in place. Then add the shelf brackets to hold it in place.
Repeat Step 5 with each additional shelf. Doing them in this order minimizes how much any shelf interferes with your ability to maneuver any given shelf.