In order to use instant transmission in conjunction with an attack, you must first "couple" instant transmission to that attack. This is declared in your update and must be done prior to entering the battle.

You may couple instant transmission with any fundamental or advanced ki attack. As of the 5.4.4 patch you may no longer couple IT with a finisher. Coupling will create a new attack called "Instant Transmission (Attack Name Here)". For example, Instant Transmission Kamehameha or Instant Transmission Big Bang Attack. This attack will be undodgable by your opponent (though it may still be blocked, no attack can be both undodgable and unblockable). You can only couple instant transmission to one attack at a time, but may change it as many times as you wish.

Keep in mind that you may use this attack once per battle or use instant transmission to dodge one attack. You cannot do both. Using this coupled attack will also count towards your limit of using that specific move on its own.