The package-layered-lambda helper now has an example how I package a custom
layer:
https://github.com/bifravst/package-layered-lambdas/commit/b837cd8059c5891f72bf647986331f8056051a1e#diff-329bb1009155d275b0fc9c22f559359c637ad878a8427fda619a0a7416eb1f20R34

Read up on it here:
https://coderbyheart.com/how-i-package-typescript-lambdas-for-aws