Stall holder applications result in a new (or updated) Contact plus a linked Stall.
When viewing a stall in Events > Stalls you will see a Send Approval button
If a stall meets your approval (or if you need to resend the email) click this button - it will mark the stall as approved and send a notification email to the associated Contact.
The email message is managed in Manager > Email Automation. Click on Market Stall Holder Approval. You can edit the Subject, and where the main Button in the email links to. In the Content tab you can also alter the email text.
When a stall holder receives the email, the main call to action will take them to the link and they will be automatically logged in, ready to place a booking.