Best way to slide into a guy's DMs?

I'm recently out of a multi-year relationship. In college, there was a guy who was interested that I vibed with a lot. I didn't pursue a friendship since I was in a relationship and I felt being friends with the guy would be risky since I was crushing on him. We haven't talked in years but would it be weird to reach out? And how would I start?
Best way to slide into a guy's DMs?
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