To-do list with Google Sheets
Google Sheets has a template called "To-do list" which is completely editable. You and your client can work on the same checklist, or you can use it to assign your client tasks which you can then view their progress. Your client can access it by simply clicking on the module.
Open Google Sheets and scroll through the template gallery to find the To-do list. Here's what it looks like:
When you share the link, make sure you set it so that anyone with the link can view, edit, or comment. If it doesn't work for your client, look at the share settings.
You can choose who can see it as well as what they can do; view, edit, or comment. Click the Share button to make changes to the settings and to get the link.
Adding the to-do list to your project
For module type, select URL. Paste the share link into the field and update the module. That's it!
Once you have the link in the project portal, you don't need to update the module again. All of your changes to the sheet will be seen when your client clicks on the module.