If you've never taken enemas before, or had only the Fleet-type disposables, it might help to think of the treatment as a form of personal hygiene rather than medication.
Remember too, that you're not likely to move all that garbage out in one or two tries; once anyting moves out, you can then take further action from above with a better diet, one action helping the other.
That being addressed, we'll assume you're administering to yourself, without a partner, and in private. You'll probably be using a corner of the bathroom, rather than in bed, so spread some old towels and make yourself comfortable. And be sure to run enough solution through the tubing to expel all the air before the nozzle is inserted.
One you're taking in the solution for the first time, don't do anything that increase your discomfort; at the first urge to expel, do so. You can always try to take on more later.
As you become more familiar with the technique, the process will seem less awkward and you'll develop more confidence in your ability to control things. You can then try large volumes, different solutions, etc.
Finally, reward yourself with a hot bath or shower at the end of the process if possible. It will contribute to a sense of cleanliness and well-being.