The number of holes in each should be the same so I would start by checking to make sure they're long enough for your levers. If they are different lengths I would and the short one fits I would cut the long one to same same length as the short one.
Once you get the hole problems figured out lace the covers like a shoe and slip them on the levers while loose. Starting from the bottom cinch the laces up (it may take several tries) until they are tight enough to keep them from rotating on your levers.
Note the direction of the laces (toward the grip or toward the levers) is a combination of fitment and preference. If they will fit without binding either direction then use your preference. Also note the check for direction and fitment should be figued out prior to tightening the laces.
Retired USAF, IYAAYAS!
2008 V-Star 1100 Custom Bobber project
2011 Crimson XR with Forged Highway Bars / Hard Bags / Passenger Backrest, Ness parts including Smooth Layback Plate Frame/Mount / Rad 3 Mirror / deep cut grips / drivers floorboards /shifter peg / brake peg / passenger pegs. Witchdoctor black rack, Lloydz filter / timing wheel / VFC III, 7Jurock windshield, Drag Specialties seat.