On mine the sleeve was an 8" corrugated tube and in the bottom of the tube was a teepee structure to hold the pole in the center. The sleeve had a welded piece of rebarb welded to it that went into the ground to act like a ground rod. I braced it in the hole and poured in the concrete. Once the concrete was set, we dropped in the pole and leveled it with some wood shims between the pole and sleeve at the top of the sleeve. Once it was perfectly leveled, slowly added play sand between pole and sleeve and used a 1" dowel and packet around - removed the top shims and checked for level - perfect. I then mixed some two part urethane I had and poured on top to seal it all up. If I ever need to remove it, I will just break out the urethane and **** out the sand with a shop vac. The pole will just pull out the help of a few friends.
I even ran a 14-2 Underground romex to the center of the pole in the event I wanted to get creative but never used it.
On a side note, the first one I ordered was a 4" pole at the base but when it was delivered it was bent. When I talked to the flag pole company, they said they would send a new one but I upgraded to the 5" which was stronger and really glad I did.