How do I properly reject an applicant???

[Jenifer Kynor]

I am not sure of what the proper protocol is for rejecting applicants.  Do you send an email as to why they are rejected? How much information do you give? Are you legally required to give a reason? Does it depend on the state you're in? 

For a while now, I've been curious on how to properly approach rejecting a tenant. Any advice would be super helpful! 

Thanks in advance!