The best way to do this is to have the seller directly invest in a separate class of stock (e.g. no carry). That keep the deal clean. Try to negotiate a liquidation preference in favour of your investors (which is market for PE deals)

It depends on the waterfall and what class of equity the seller has. If the company post-close has a preferred class of equity (investor equity), you will need to determine whether the seller's rolled equity will be of the same class, of a more junior class (but still above the carry), or of the same class as the carried interest. If it's more senior, you attribute a value to it (likely a % of the deal value) and the seller will presumably receive that value in distributions before you get your carry (and then the seller will share in subsequent distributions pro rata based on the % you agree on). If it's of the same class as the carry, the easiest way to explain it is just as a % interest after your investors receive their capital back. You don't need to describe it in terms of carry, it's really just your ownership interest that you have earned and will earn after the preferred get paid.