We should work on script metadata early for a Unicode version, so that it is available for tools (such as Mark's "UCA" tools).
If the new Unicode version's PropertyValueAliases.txt does not have lines for Block and Script properties yet, then create a preliminary version. Diff the Blocks.txt file and UnicodeData.txt to find new scripts. Get the script codes from http://www.unicode.org/iso15924/codelists.html . Follow existing patterns for block and script names, especially for abbreviations. Do not add abbreviations (which differ from the long forms) unless there is a well-established pattern in the existing data.
Aside from instructions below for all script metadata changes, new script codes need English names (common/main/en.xml) and need to be added to common/supplemental/coverageLevels, under key %script100, so that the new script names will show up in the survey tool. For example, see the changes for new Unicode 8 scripts.
Can we add new scripts in CLDR trunk before or only after adding them to CLDR's copy of ICU4J? We did add new Unicode 9 scripts in CLDR 29 before adding them to ICU4J. The CLDR unit tests do not fail any more for scripts that are newer than the Unicode version in CLDR's copy of ICU.
We need sample characters for the "UCA" tools for generating FractionalUCA.txt.
Look for patterns of what kinds of characters we have picked for other scripts, for example the script's letter "KA". We basically want a character where people say "that looks Greek", and the same shape should not be used in multiple scripts. So for Latin we use "L", not "A". We usually prefer consonants, if applicable, but it is more important that a character look unique across scripts. It does want to be a letter, and if possible should not be a combining mark. It would be nice if the letters were commonly used in the majority language, if there are multiple. Compare with the charts for existing scripts, especially related ones.
Google Spreadsheet: Script Metadata
Use and copy cell formulas rather than duplicating contents, if possible. Look for which cells have formulas in existing data, especially for Unicode 1.1 and 7.0 scripts.
Problems are typically because a non-standard name is used for a territory name. That can be fixed and the process rerun.