It will depend a lot on how the equity is currently structured.
Here is the way you should structure it between yourself and the other co-founders:
Question for you:
Is the equity they're giving you diluting their equity structure or coming out of a remaining pool? For example: if they started with 40%/40% each and set aside 20% for employees, advisors or another co-founder later.
Here's a generic document on Docracy that you can customize for you needs:
This term sheet is for use in connection with the Founders' Agreement Template. The founders can agree in principle on what the basic business terms of their relationship will be, should the project prove successful. But postpone the details of a definitive agreement until it becomes necessary. If that never happens, then this term sheet will become moot. All of these terms are subject to negotiation with other parties if the company admits new founders, or raises outside capital. But having the terms outlined in advance will smooth the path of future discussion.