Notify them that the debate starts in Round 1, they should use the description section to define terms and outline what their position is and what position a prospective challenger is expected to debate. A debate argument should be formatted with paragraphs, use subheadings to denote arguments and use inline citations whenever sources are being cited. Debaters should be conscious of the reader's perspective, as they are the voters. Making things easier for the reader goes a long way towards presenting a professional argument. You could include little things like placing rebuttals before arguments or summarising the opponents arguments succinctly if rebuttals are being placed after so the reader doesn't have to refresh their memory by scrolling up. For technical debates you could recommend or link to one of the standardised essay formats. All arguments should be proof-read for spelling and grammar as well as conciseness.
Etiquette includes things like ad hominems, not posting images of text to circumvent character limits, not using comments section to store arguments or sources without mutual agreement, formulating a decent RFD when voting etc. Newbs should be advised against Votebombing, how to report poor conduct, vote trading is not acceptable etc.
If you wanted to go one step further you could include fundamental debate philosophy that all debaters should be aware of. Fallacy for example is best avoided by conceptualising one's argument as a syllogism and identifying if the conclusion follows strongly from the premises. If not, the argument is either weak or fallacious.