For example, to connect to freenode using znc:

/network add freenode-znc
/server add -network freenode-znc -auto -tls us9.ircnow.org 31337 user123/freenode:pass123
/save
/connect freenode-znc

Remember to replace user123 and pass123 with your username and password.

In ~/.irssi/config, inside servers, add this block:

servers = (
  {
    address = "us9.ircnow.org";
    chatnet = "yournetwork";
    port = "31337";
    password = "username/yournetwork:password";
    use_tls = "yes";
    tls_verify = "yes";
    tls_cafile = "~/.irssi/certs/CAs.pem";
    autoconnect = "yes";
  },
)

Suppose you want to connect to freenode. Here is what that block would look like:

servers = (
  {
    address = "us9.ircnow.org";
    chatnet = "freenode";
    port = "31337";
    password = "username/freenode:password";
    use_tls = "yes";
    tls_verify = "yes";
    tls_cafile = "~/.irssi/certs/CAs.pem";
    autoconnect = "yes";
  },
)

Notice that the address should still be us9.ircnow.org, because you want to connect first to the bouncer, then have the bouncer connect to freenode.

If you have issues with SSL verification, make sure you have CAs.pem located in the right folder. On OpenBSD 6.6, you would do this:

$ mkdir -p ~/.irssi/certs
$ cp /etc/ssl/cert.pem ~/.irssi/certs/CAs.pem

To connect to multiple networks, add more blocks:

servers = (
  {
    address = "us9.ircnow.org";
    chatnet = "freenode";
    port = "31337";
    password = "username/freenode:password";
    use_tls = "yes";
    tls_verify = "yes";
    tls_cafile = "~/.irssi/certs/CAs.pem";
    autoconnect = "yes";
  },
  {
    address = "us9.ircnow.org";
    chatnet = "ircnow";
    port = "31337";
    password = "username/ircnow:password";
    use_tls = "yes";
    tls_verify = "yes";
    tls_cafile = "~/.irssi/certs/CAs.pem";
    autoconnect = "yes";
  },
  {
    address = "us9.ircnow.org";
    chatnet = "oftc";
    port = "31337";
    password = "username/oftc:password";
    use_tls = "yes";
    tls_verify = "yes";
    tls_cafile = "~/.irssi/certs/CAs.pem";
    autoconnect = "yes";
  },
)

You will want to add the networks also in the znc webpanel. Please see the instructions for how to do that.