An overlay is a file containing very simple logic to test whether we are currently building a specific pull request or git branch (useful so that overlays work on your own fork) and which changes some of the variables whose default can be found in ci-basic-overlay.sh.
The name of your overlay file should be of the form five_digit_PR_number-GitHub_handle-branch_name.sh.
Example: 00669-maximedenes-ssr-merge.sh containing
if [ "$TRAVIS_PULL_REQUEST" = "669" ] || [ "$TRAVIS_BRANCH" = "ssr-merge" ]; then
mathcomp_CI_BRANCH=ssr-merge
mathcomp_CI_GITURL=https://github.com/maximedenes/math-comp.git
fi