Please don't. Every time I see a question about MERGE, I scramble for the links I post to urge people to change their minds. So I put them together here.

If you insist on using MERGE, the biggest TL;DR is:

Please ALWAYS use HOLDLOCK on the target.

But for a long list of details, see these resources:

• Use Caution with SQL Server's MERGE Statement
  This is the one that occasionally ruffles feathers, but I talk about some real shortcomings with MERGE and some bugs that have never been fixed.

• An update on MERGE
  Largely in response to that post, Hugo Kornelis recently investigated many of the issues I brought up initially (and a few others), and concluded that most are no longer issues in current versions of SQL Server. Shockingly, I left a comment.

• Be Careful with the Merge Statement
  Michael J. Swart initially advised you be careful with MERGE.

• What To Avoid If You Want To Use MERGE
  In a follow-up post, Michael pulls no punches here, except the punchline: his now stronger opinion that you just avoid MERGE altogether.

• Please stop using this UPSERT anti-pattern
  When I do succeed in weening someone away from MERGE, they often turn to a problematic UPSERT approach.

• SQL Server UPSERT Patterns and Antipatterns and Mythbusting: Concurrent Update/Insert Solutions
  Michael also talks about how to perform concurrent updates/inserts.

• UPDATE if exists else INSERT or Solutions for INSERT OR UPDATE on SQL Server
  Probably the most canonical answers I have that explain why you should read all the links above.

• Some MERGE bugs from Paul White
  Paul goes into great detail about several bugs he's discovered with MERGE over the years.