In order to engage with Mirri, talking to her will initiate a conversation and you can select the Companion Menu option.
From here you can choose two different customizations. At the very first character sheet on the right side, underneath the Combat Level, is the Outfit. Here the player can choose between No Outfit and Companion Outfit.
If the player decides to go with No Outfit, the current equipment that Mirri is wearing will be displayed. If a player wants to have a costume that they own worn by Mirri, they have to choose the Collectibles icon on the top right. Here a player can also set the mount that Mirri is supposed to ride.
In order to have their own customized Outfit displayed, the player needs to go to an Outfit Station that is not in a player's house because Mirri needs to be active at the time of customization.
In the Outfit Station menu, the player can choose between themselves and Mirri. From here it works the same as with a player Outfit. You can also choose colors for both the Outfit or a Costume at this station.
The head armor will be hidden at all times and will not be displayed regardless of equipment or cosmetic override settings.
Depending on gender, race or other restrictions, some of the player's cosmetics might also not be usable by the companion.
Players can't use polymorphs, personalities, skins, hats, hair or tattoos on companions.