The Sonic R Curse is the most commonly used way to summon the Tails Doll. It involves drawing the Tails Doll out from a cursed copy of Sonic R. Performing the Curse depends on whether you're using Sonic R for the PC and Sega Saturn, or on Sonic Gems Collection for the Gamecube and Playstation.
CurseFor the Sonic R on the Sega Saturn/PC, you must complete Sonic R 100%, then the screen will show Tails Doll "sitting" on a red and white checkerboarded floor. For Sonic Gems Collection, you must complete the game 100%, then play Tag Mode as Tails Doll and exactly in this order, tag Metal Sonic, EggRobo, Metal Knuckles, and lastly, Super Sonic. Then Tails Doll will be shown again on the checkerboarded floor.
At the final stages of the curse, you have alerted the Tails Doll. What happens next is entirely up to it.