All of the instructions for contributing from the main repository apply, and won't be repeated here. You simply need to change references to caskroom/homebrew-cask (that repository) to caskroom/homebrew-versions (this repository).
For Casks of nightlies, version, sha256, and url change frequently, making the Cask constantly outdated. To alleviate having to regularly update them, the use of a url do block in conjunction with version :latest and sha256 :no_check is encouraged, to ensure the latest version is always fetched.
See this pull request for exist-db-nightly for an example of the procedure.
Example (exist-db-nightly.rb):
url do
require 'open-uri'
base_url = 'http://static.adamretter.org.uk/exist-nightly'
builds_url = "#{base_url}/table.html"
latest_build_filename = open(builds_url) do |io|
io.read.scan(%r{<tr>.*?<td>(.*?)</td>.*?<a href="([^\"]+)">dmg}m).max[1]
end
"#{base_url}/#{latest_build_filename}"
end