Taking them to pieces is always the easy bit. It should just slide gracefully in, assuming that you are using the same halfshaft that came out.....
Often though, it is a case of jiggling/wiggling it around gently, as it can miss the 'ole, so to speak, by just a fraction - you haven't got a lot of leverage left on the last inch or two of the remaining bit to insert and, particularly if it is the longer halfshaft a small movement at the hub end gets amplified into a much larger movement at the other end.
One way that can help, if you've got a spare pair of hands and the wheels are in the air, is to get someone to turn the diff slowly as well, which will help the shaft to engage on the splines, whilst you feel delicately for whether the shaft is in the right alignment. However, tempting it may be, don't use a hammer beyond a light tap or two onto a piece of wood held against the hub when you think it is in the right place. Good luck
MG Mark