Welcome to the community. 
There’s an unofficial Steam Deck guide channel in WeMod’s Discord community → https://www.wemod.com/discord.
An unofficial guide has also been provided in text format by a member of the community, here: → My perfected and streamlined WeMod tutorial for Steam Deck
Though it’s possible to use WeMod on Steam Deck, the WeMod development team itself does not officially support the Steam Deck, so any official support we can provide is limited. Discord is the best place to ask, as other community members may also be using WeMod on the device.