At its core, a Simple Agreement for Future Equity (SAFE) is essentially a convertible debt instrument for financial modeling purposes, but without all the features associated with debt. For example, a SAFE typically will not have an expiration date or bear interest, but the principal amount will convert into equity at some point in the future, potentially at a discount to the next round, and this conversion may be capped at a specific valuation. To model a SAFE, simply add a debt financing with 0% interest and the features outlined in the deal. For more on the accounting treatment of SAFEs (i.e. equity vs. debt), this guide is excellent.