It's really a matter of making up a story, which is what we're all doing anyway. You'll need, of course, to collaborate with your DM on it. And it can be extremely complex, or very simple, or somewhere in between.
Old Ones are enigmatic and just do weird things. Fiends and Fey both are known to play tricks and entrap people and use them for their own purposes.
For example, in the case of one of my characters (who started as a fighter), he had rolled a trinket in his starting gear, which happened to be the "small idol depicting a nightmarish creature that gives you unsettling dreams when you sleep near it". Since I wanted to have a pact with an Old One anyway, that made for a simple hook. At one point my character decided to spend a long rest meditating on the figurine. It came to life, burrowed into his body, and fused itself onto his spinal cord, and that was his link and became his pact.
The DM was fine with that, and voila, a Warlock!