Interior walls are rarely square, especially in closets. There's a tool called a bevel gauge that you would use to get the angle of the inside corners. You can pick one up for about $10.00 or so. Start on one side and set the guage to whatever angle the walls are. Take that angle and pencil it on the shelf. Measure the back wall to the other side, subtract an 1/8" of an inch and mark just the width on the back edge of the shelf. Take the bevel guage and set it to the angle on the other wall. Draw the line on the other end of the shelf.
Cut the two sides of the shelf. Make sure you mark a "L" and"R" and"F(front)" on the shelves to get the correct shelf into it's correct place. Multiple shelves will require numbering as the angles will not remain constant from top to bottom.
The shelves might need individual fine tuning from front to back if the walls are not consistant.
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