Excel would be a great tool for it if you are not using any other test management tools already.
Divide the application into features and sub features. You can dedicate one sheet per feature and list down all of the sub features inside that sheet. Also track the automation status in the sheet as a seperate column.
You can add a column to track the automation as well.
There are some freely available test management tools like Testlink . Just try them out to see that it suits your need or not.
I use this with my team. We love it. You can manage requirements and test plans, can link requirements directly to test cases to show where your validation holds up, and it has APIs to plug into automated test tools. It’s PHP and open source so you can make changes if you want.