There are a few options to integrate locize into your code base:
Just open the "latest" version of your project by clicking on the version latest in the Versions card.
Choose the "Import File" below in the "..." more dropdown on the languages section.
Make sure to add all desired languages.
On the version overview page (having opened a version, i.e. the "latest" version), click on the "+" button to add other languages beside your chosen reference language.
You will have to open your source language below and add your first namespace (resource file) on the next page.
Now click on the created namespace in the "Start to translate" card.
To add a new segment/text click the ADD button.
Enter a segment key and a segment value and click ADD.
Click the SAVE button to confirm your work.