Beastforms are a feature specific to the Druid class.
In your characters Feature tab you will find the Beastform class feature.
If you click the Beastform button, you will be able to select your beastform from the pop-up dialog:
Make a selection and click the Transform button.
You will get another pop-up asking you to confirm the action cost:
Once you click Confirm your character will get the Beastform Transformation active effect.
If the tokens for each beastform have been set up in the beastform compendium your token will automatically change to the one matching the selected beastform.
If you click the Beastform button again, the effect will end and your token will return to the previous state.
Legendary Beast
for Legendary Beast (Tier 3) select the Legendary Beast option and then drag another beast form into the field below the description.
Then you can click the Transform Button
Legendary Hybrid (Tier 3)
Select the Legendary Hybrid option, then add two Tier 1 or Tier 2 beastforms into the fields below.
Then select four advantages and two features from the available options to enable the Transform button:
Mythic Hybrid (Tier 4)
The Mythic Hybrid works the same as the Legendary Hybrid above, you just get more choice and have to combine 3 Beastforms:
Mythic Beast (Tier 4)
The Mythic Beast works the same as the Legendary Beast above, it is just stronger.
Custom Token Art
Please import the beast form item that you want to set a custom token for from the compendium into your world, and then edit the item:
They will automatically be applied when the appropriate beastform transformation is active.
The world beastform item of the same name will take priority over the compendium entry.