If you’re trying to raise the lift, avoid applying your full body weight to the seat when sitting down. Instead, perch slightly on the edge of the chair, then wrap your hand around the lever and pull all the way up. You’ll feel some resistance at first, but just keep pulling until you can’t go any further.
If you’re trying to lower the lift, sit all the way back in the seat, with your back against the backrest.
When trying to lower the lift, use your body weight to "bounce" slightly up and down on the seat.