Right now for Pharo 7 you will need to download image from here:
And because of a Monticello issue you will need to have your clone in image working directory, in the folder named 'pharo-core' (the other alternative is to reload all packages).
Then you need to switch this repository to the particular commit that was used for the image bootstrap. You can get the hash with:
SystemVersion current commitHash
...and this cannot be done now from Iceberg. Then you can create issue branches and pull request.
One goal of the sprint can be to make this all easier ;-)
Cheers,
-- Pavel