There is a small sidebar in Level Up's Adventurer's Guide on how to design a mixed heritage character. To create one, all you really need to do is pick a heritage and a heritage gift from another heritage.
If you wanted to create a Half-Elf that bears a resemblance to the Half-Elf in 5e's PHB, you pick the Elven heritage and a human heritage gift. So, you would get all of the traits commonly shared among elves-age, size, speed, Darkvision, Fey Ancestry and Trance. But instead of getting an Elven heritage gift (Mystic Rapport, Prescient Vision, and Preternatural Awareness), you would choose a Human heritage gift such as Diehard Survivor, Ingenious Focus, or Spirited Traveler.
Otoh, if you wanted a Half-Elf like Aragorn from LoTR, you pick the Human heritage and then pick one of the Elven heritage gifts I mentioned above.
Level Up's heritage gifts work much like a 1st-level feat. And then at 10th level, you gain a paragon gift that either gives your character something or goes on to improve your heritage gift. Unlike the heritage gift, paragon gifts cannot be mixed-and-match between heritages. A Half-Elf in the first example, for instance will level up and eventually get an Elven paragon gift.
After heritage, your Half-Elf character can either be raised in one of four Elven cultures (Eladrin, high elf, shadow elf or wood elf) and gain that particular culture's mechanical traits, or they could grow up in a non-Elven culture.