Get a dildo the same size and practice, practice, practice.
Lots of saliva. Don't try to start deep throating right away. Work up to it gradually and let your saliva glands kick into overdrive.
Take as much as you can without gagging. Take a deep breath brought your nose since your mouth is full. Then just relax and swallow, like you are eating or drinking. It will slide right down. You will probably only be able to keep it down for a second of two. Slide it out and repeat the process. Try to keep it down longer. As you get better, try sliding it up and down an inch or two while it's in your throat for a few seconds.
This is what worked for me. Was able to have an escort throat fuck me with his large 9" recently. I could not have done that two years ago.