I posted a howto specifically for combining a self-hosted WireGuard VPN with NextDNS for adblocking while on the VPN itself, and you can find that here: https://blog.paramdeo.com/2019/07/03/setup-an-adblocking-vpn...

However, if you're just trying to setup NextDNS you can signup for free on their website and within the account dashboard you'll find instructions for configuring your desktop and mobile OS, DoH in Firefox, router, etc. (screenshots of what this dashboard looks like are in the blog post if you're curious).