Assuming you are talking about a differential:
A crush sleeve is used to set preload on the bearings. The bearings and their races need to be properly seated in the housing before checking or setting preload. To do this, first install the carrier into the housing and lightly tap it into place with a soft mallet. Do not use a hardened tool or anything that could mar the bearing surfaces. With the carrier in place, install each of the bearing caps snugly by hand until they bottom out on the shoulder of the housing bore. Be careful not to cross-thread them. Once all of the bearing caps are in place, lightly tighten all of the cap screws until they are just snug. At this point, you can remove any shims that were installed previously (if any) and save them for later use.
The next step is to install your new crush sleeve (or reuse your old one if it's still in good condition). The crush sleeve needs to be driven onto the tapered end of the pinion shaft until it bottoms out against the shoulder on shaft itself. This may require using a drift punch or other similar tool to get started, but be very careful not to damage either surface.