At the Training Grounds, you will see four slots for training pets. At this time, there is no way to gain more training slots.
To start off, they will be empty, but once you send a pet to train, the slot will show the pet's image, as well as how much time is left in their training.
Click on one of the plus icons and select the creatu you'd like to train. From there, select whether to train your pet in Health, Defense, Power, or Agility.
Four subclasses are offered for each stat. Different training sessions require different tokens to complete. As far as I can tell, each creatu is unique in which tokens it needs for a class, and how many. No two pets, even of the same species, require the exact same Training Tokens for a class.
Quick classes take 6 hours to complete. They cost 5 Training Tokens and increase a stat by +1.
Casual classes take 10 hours to complete. They cost 9 Training Tokens and increase a stat by +2.
Intense classes take 16 hours to complete. They cost 15 Training Tokens and increase a stat by +4.
Extreme classes take 24 hours to complete. They cost 25 Training Tokens and increase a stat by +7.
Though they take both the longest amount of time and the most amount of tokens, Extreme classes are the most cost-effective class in terms of stat gain.
Training Tokens can be gained several different ways.
The primary method of earning tokens is through Harlow's Donation Drive, which can be found under Activities. Harlowe gives you 30 minutes to hand in 1-4 items found in Rescreatu's NPC Shops, in exchange for a random Training Token and a random TU token. Keep in mind, though, you can only take 25 quests for Harlowe per day.
The secondary method is buying them from merchant shops. Type the name of the token you're looking for into the search bar in order to find the cheapest price. You may also randomly find tokens in Relcore's Trash Isthmus, from which you can take one item per day.
The tertiary and final method of gaining tokens is through the Wishing Well. Located on the Training page, you can put in two tokens to receive another colored token of your choice.
If you know any other ways of gaining stats or training tokens, feel free to submit them to me.
In the meantime, happy training!