While stopped stand over the bike and clip and unclip one foot at a time while keeping your other foot on the ground to hold and balance you and the bike. Do this for awhile until you can clip in and out without struggling. You might also want to ride with only one foot clipped in and practice clipping in and out the other foot while coasting and coming to stop signs. Just be certain to not lean toward the side of the bike that you are clipped in. I have been riding for a long time and occasionally almost lose my balance while stopped and clipped in to only one pedal.

Remember, clipping and unclipping should be done at the absolute bottom of the pedal stroke which is where you have the most power because of leverage and can achieve the maximum bending of your ankle/leg.