You can just post a link with the normal toiurnament rules and regulations and most of the people participating know what's to be expected and know better. So none of this is that big of a deal.

And if you expect people to join your group and participate in the smash tourney, this won't be more than 20 people (Which would be boring and take less than a couple hours).

In case you want to be more familiar with normal tournament rules and regulations, you want to be as specific as possible. Meaning, with these rules in place:

Items are allowed.
How can you tell who uses meta knight and how can you regulate if they only use him once?
How will the players battle?

Etc. etc. More experience comes from hosting more tournaments. Seiing as this is your first, this should be rather small. If there's at least 12 participants, medals will be rewarded to the top 4 winners.