If you have suspended wooden floors and cavity brickwork there should already be air-bricks installed. Air-bricks (vents) will not cure or prevent damp under the floor. They will only vent the space that exists there thus helping by evaporation. If you install vents below the DPC the existing cavity will be filled with a weak concrete mix up to two courses below the DPC.(building regs) If you install vents above the DPC and the cavity HAS been insulated no sleeving is necessary because the insulation restricts any air movement.

I want to install some air bricks to help cure/prevent damp under my floor but the walls are cavity walls. How do I go about doing this?

Thanks.