create a new file in this directory, its name must follow this pattern vscode.proposed.[a-zA-Z]+.d.ts
creating the proposal-file will automatically update src/vs/platform/extensions/common/extensionsApiProposals.ts (make sure to run yarn watch)
declare and implement your proposal
make sure to use the checkProposedApiEnabled and/or isProposedApiEnabled-utils to enforce the API being proposed. Make sure to invoke them with your proposal's name which got generated into extensionsApiProposals.ts
Most likely will need to add your proposed api to vscode-api-tests as well