Guidelines
First of all, if you would like to allow the users to translate offline, you need to have an Administrator role in order to enable such an option in Configuration → Settings → Translation → Workflow and access → Linguist general options → Allow linguists to download / upload.
2. Create a project in XTM (using any supported file extension).
3. Generate the XLIFF file in Project editor → Files → Offline translation.
4. After you generate the XLIFF file, you can download it using the green arrow.
5. A generated XLIFF file can be translated in any text editor or be used as a source file in another CAT tool that supports this format.
6. Finished translation file can be then uploaded back to XTM again in Project editor → Files → Offline translation, using the purple arrow this time.
At this stage, you can decide on the workflow step you would like to upload your translation to as well as specify what segments shall be approved straight away:
IMPORTANT!
Please, note that if the file was translated in another CAT tool, it is important to maintain the XLF extension.
Should you not want to translate the project outside XTM, you can always use the XLIFF file as a source file for the XTM project and translate it in the XTM.
Good to know!
In order to learn about certain restrictions as well as good practices when dealing with offline translations, please, make sure you familiarize yourself with the respective article: Offline translation - restrictions and good practices for XLIFF and multi XLIFF.