Lay the upper receiver/barrel assembly upside down (sights down) on a table. Look at the bolt’s right side; you will note a long shiny piece of steel with a crescent shaped front. This piece is spring loaded and projects rearward.
With the forend positioned approximately in the center of the mag tube, slide the bolt into the receiver until the front portion falls downward and into the receiver’s locking recess. Slide the forend rearward until it mates with the front of the bolt.
Looking at the bolt from the rear, at the bottom right you will note the rectangular projection of the spring loaded “shiny piece of steel” you viewed earlier. Push that rectangular spring loaded piece forward while pushing the forend rearward, and they will snap together.
DO NOT USE TOOLS, AND DO NOT FORCE ANYTHING. That beauty’s parts will go back together like a knife through butter.
HTH