Sit upright at the end of the bench, stand up with them and heave them onto your lap as you sit back down. Then just lie back, kick them up a little and bend your arms at the elbow. They'll naturally fall onto your chest. Lifting off your chest in the first place is harder than the first few reps, but if you can't do that then they're probably going to be too heavy for you to get more than 3 or 4 reps out of anyway.
That's how I do it anyway.