Do what you are best at. Click on "Edit on reference", this will bring you to locize.
You can choose how to login to locize.
If you have a locize account, you can use that one or you can login with your localistars credentials. For this just click on the localistars logo and enter your localistars credentials.
Once signed in, you're going to be automatically added to the appropriate locize project.
Having the translations done, go back to localistars and submit the job. This way your client knows, you've done the job.