Actually there is no easy way to learn how to sit in seiza, but you can work on getting your ligaments and tendons streched and getting the whole joint more flexable.
one thing is sitting on the floor, feet forward, take one foot on top of the other leg (thigh) and hold it with your hand. strech it back and forth, strech each finger, hold your foot on the brige (the side of the sole) the other hand on the ankle, as safty, and turn it around. speed is not that important, reach maximum circular movement. do this to both sides (the circles, as well as both legs)
straighten your legs, put your hands on the floor at the sides of your body for support. bring your feet and fingers towards you (this is called dorsi flexion) as if you would walk on your heels when standing. hold for a while and release, do the same but pointing your fingers forward (called plantar flexion) as if you would walk on your finger tips.
except for that, sit in the aikido possition from time to time during the day, just for a minute, and more as time goes by. this way you will flex the joint without danger of taring anything.
remember, it is a long prosidure. for some it is easier, for some it isn't, but with time you can get better