When Mass is celebrated more solemnly, it is customary to incense the altar during the Introit. This may be done while the choir is singing the Introit, or after the celebrant has finished singing the Introit.
A diagram showing the method of incensing the altar can be found in a variety of liturgical books. There is one method for incensing an altar that is against the East wall, and another for a free standing altar. The difference being that the celebrant walks completely around the free standing altar.
Assuming that incense will also be used at the Offertory, only the celebrant is incensed at this time. The celebrant, other ministers and the people are incensed at the offertory.