I want to date a couple, but don't know how to approach it.

I have an interest in being a third; there has been more than one instance where I've had an interest in couples I am friends with. Like a specific interest in dating both and not wanting to break them up. But I really have no idea how to approach trying to become a third in a relationship, and it can also be quite stigmatized among monogamous bisexuals which is a bit daunting.

It's not entirely clear from your message whether you want this as a generalized relationship framework, or whether there are specific couples you know who you want to date.

If it's the former - if you're just interested in finding a couple to date - you are in some serious luck. That is something that a LOT of couples want, and you will not have a hard time finding a couple to date! A third person who dates a couple is called a "unicorn," and couples seeking one are called "unicorn hunters." You can read more about this on my FAQ page here!

However, that does mean you will need to do a lot of work to screen people and give yourself permission to say no. It's just like monogamous dating - don't date the first person who likes you. Meet lots of people, know your limits and boundaries, and make sure you end up dating someone who meets your needs and fits your personality. 

If there are specific couples in your life that you'd like to date, it works just like crushing on anyone else. Do what you can to suss out their interest - try bringing up triads/unicorns/non-monogamy and see if they have any general thoughts on it - but in the end, the only way to find out is to ask! It can be frightening to approach people about something that's so misunderstood, but if the couple seems open and safe, the worst they can do is say no!