gistfile1.txt

/server add <name> <team>.irc.slack.com/6667 -ssl -password=<password>
/set irc.server.<name>.ssl_verify off
/set irc.server.<name>.nicks <username>
/connect <name>

weechat-slack-irc.conf

<name>.addresses = "<server>.irc.slack.com/6667"
<name>.proxy
<name>.ipv6
<name>.ssl = on
<name>.ssl_cert
<name>.ssl_priorities
<name>.ssl_dhkey_size = 512
<name>.ssl_verify = off
<name>.password = "<password>"
<name>.capabilities
<name>.sasl_mechanism
<name>.sasl_username
<name>.sasl_password
<name>.sasl_timeout
<name>.autoconnect
<name>.autoreconnect
<name>.autoreconnect_delay
<name>.nicks = "<username>"
<name>.username
<name>.realname 
<name>.local_hostname 
<name>.command 
<name>.command_delay 
<name>.autojoin 
<name>.autorejoin 
<name>.autorejoin_delay 
<name>.connection_timeout 
<name>.anti_flood_prio_high 
<name>.anti_flood_prio_low 
<name>.away_check 
<name>.away_check_max_nicks 
<name>.default_msg_part 
<name>.default_msg_quit 
<name>.notify 

You can do it all in 2 lines:

/server add <name> <team>.irc.slack.com/6667 -ssl -password=<password> -ssl_verify=off -nicks=<username>
/connect <name>

Thanks for the gist. There are not so much documentation about how to do it.