Step 1: Add a role in Discord named Muted
Step 2: Pick the Channel category you want the user to be muted in
Click "Edit category". (You'll need to do this for every category you want the user to be muted in.)
Step 3: Set the Muted role to deny "send messages" permission
Add the Muted role by clicking the small + button. Then set Send Messages permission to deny.
Step 4: Create the /mute slash command
Create a new flow using the "Slash Command" trigger. Name the command /mute.
Add 1 option that is a User type and name it member.
Step 4: Add the Muted Role
Use the Add Role action to add the Muted role to member that was given in the slash command.
Step 5: Reply to the command
Send a reply for the command saying that the user has been muted.
Step 6: Turn on the flow and test!
Step 7: Create an /unmute command.
Do the same thing you did for the mute flow, but instead of ADDING the Muted role, REMOVE the Muted role and name the command /unmute