Instructions
Choose a character that is particularly adept at submissions, such as Antonio Rodrigo Nogueira at heavyweight or Demien Maia at middleweight. This will make submissions much easier to pull off as you get used to the timing.
Pick an opponent that has very low submission defense, such as Mustapha Al Turk at heavyweight or Drew McFedries at middleweight. The computer can be very difficult to submit, and an opponent with low submission defense skills increases your odds of a submission victory.
Set your difficulty level at "Beginner."
Strike the opponent's body as much as possible early in the fight with body kicks from a distance. The basic kick button works well for this since most fighters will kick the body automatically from mid-range. Use the right stick to initiate a clinch and throw knees to the body with the kick buttons to further damage the torso. This lowers the opponent's stamina, which makes a submission much easier to execute.
Execute a takedown on the opponent using the right stick and either the left trigger or the left shoulder button. A slamming takedown works best since it takes away extra stamina from the opponent, depending on your fighter.
Click the right stick in when you are on the ground and the opponent's stamina is low. This will initiate the submission. The specific hold will vary depending on your fighter and the specific position he is in on the ground. Hold the stick in for a second or two to "charge" the submission and then release the stick.
Rotate the right stick either clockwise or counter-clockwise in smooth circles as soon as you let the stick out from the "pushed in" position. When done correctly, this will finish the submission and lock it up to where the opponent taps out. If he escapes, let your stamina increase and then try it again.