Constitution is almost always trained through combat. Any of the combat styles, melee, ranged, or magic can be used to raise constitution.

Historically, constitution was trained by skillers (accounts that wish to remain at the lowest possible Combat level) because it did not contribute to the combat level calculation. However as this is no longer the case, there is very little reason for any account to train constitution without training other combat skills at the same time.

Constitution is generally considered to be one of the easiest skills to train, as it is trained whenever the player is in combat.