You must be able to lock your knees against the inside of the deck or against thigh braces while the balls of your feet brace firmly against fixed foot braces. If your kayak has sliding footbraces, you'll need to immobilize them. For comfort you may wish to glue minicell foam pads where your knees and thighs contact the inside of your kayak. Check your kayak is strong enough to withstand your weight rolling in it on the ground. Plastic whitewater kayaks are great for learning the roll, and they should withstand the rigors of repeated land practice.
Composite kayaks are more liable to damage.
Some sea kayaks are quite difficult to roll and require good technique. You'll find it easier to master the techniques first in a kayak that rolls easily. Dry Roll Animation Find a soft area of ground , or set your kayak on a mat or carpet. Sit in your kayak holding your paddle. Grip the kayak with your legs.