The last step of the Ripley challenge, in the Alter Ego mission, will ask you to Enter a Slurp vat while wearing the Ripley outfit. You will find the huge tanks in the heart of the Slurpy Swamp central factory.
All you have to do is break the lid of the tank, to find yourself in the blundering, and succeed the challenge.
Map
The only condition to validate the challenge is to wear the Ripley outfit. It unlocks at level 20 of your Seasonal Battle Pass. You must also have completed a total of 4 missions beforehand.