A step by step guide using images to help.
Go to ACP > click Posting >

Click add a new BBcode

1 add the bbcode usage code
2 add the HTML replacement code
3 add the helpline
4 click on display on posting page
5 press submit

Your page should look something like this

If you installed everything correctly this page will show

You will now have your new bbcode displayed on the posting page

With most custom bbcodes you will need to add text between the bbcode tags, with this example you paste the video code between the tags

End result
