Culture of your organization is one of the most important, but also most difficult thing to "manage." In reality, you can evolve and influence, but can't really directly control your culture. It's almost an emergent phenomenon that surfaces from the interactions between people (especially the founders and early employees) and incidents (wins and losses) that get engrained in our collective minds.
But before we talk about culture, let's first think about your core values. Why should companies have core values? A practical reason is people have different set of values. As you have more people joining your organization, your values will start to diverge. As in all forms of sustainable and adaptable structures (such as organisms or organizations), there has to be a balance of convergence and divergence. If the balance tips too much towards one way or the other, the organization will face inevitable state of death. To avoid this fate, core values can give you the convergence, while culture being additive can give you the divergence.
Core values give your organization the power of convergence. If one founder believes you need to work 100-hour weeks to achieve something great vs another beliving maintaining 40-hour week and giving people work-life-balance is the only way to true sustainability, you can probably argue and find data to justify your beliefs or values. But if you have this dichotomy within your group of founders, it'll be incredibly difficult to make it work. Another example would be if one founder believing in customer above everything else (including employees) vs another believing in employee as the highest priority vs another believing in shareholders as the utmost importance. Now one may not necessarily be exclusive (an employee may be a customer in case of people-related software, or if everyone has shares/options, everyone also become shareholders), but having a clear core values telling which is of the highest priority, it makes it less stressful for everyone to argue, as it is set the moment you apply and walk in the door of the organization.
This does not mean core values never change, but they evolve very slowly (as they should). They are there to give you guidance towards the set of beliefs and priorities that help align everyone in the organization over a long period of time. They are used to make decisions across all dimensions of business, so it makes sense to invest time in getting them setup and communicated clearly.
Don't expect your core values to be perfect nor have full coverage of all aspects. But focus on what's important to your organization more than other places, what makes your organization unique in a way that you all believe is important and needs to be emphasized.
Try to crystalize them into 3-7. Ideally, fewer the better (as they need to be talked about, executed with, and celebrated for) so it becomes memorable. You want to build it with your core teams, so that everyone becomes the ambassadors and evangelists within the organization. Once you have them written out, apply them everywhere.
Recruiting, onboarding, product decisions, promotions, customer decisions, and even letting go of people. Then collect the stories that best represents your core values. You need to weave those stories and tell them again and again. So that they become the values that your organization rallies behind.