Here's how you can do it:
First, put one hand on the kayak's side closest to you, and put your other hand on the opposite side — so your arm will be extended across the seat opening.
Pull yourself up onto your kayak so that your belly button is over the seat. Your legs will still be about halfway in the water.
Next, twist around to get your bottom on the seat. Your legs will still be dangling in the water, but you should have most of your body in the kayak now.
Make sure the kayak is stable, pull your feet in and put them in front of you.