Mostly raids against the civilian population (the English called these chevauchees in the Hundred Years War). The aim was both to damage the other side's resources and pressure the local nobility into defecting to your side. Each noble or town that came over expanded your zone of control and diminished that of the enemy. This involved lots of minor skirmishes, perhaps some attacks on fortified positions if these looked vulnerable, and possibly a siege against some key position. In western Europe 1100-1300 it involved lots of negotiation, intercession by third parties (the Pope, maybe the Emperor in the German lands, or the French King before 1200), assertions of right and so on.