You will need to install the app on your Shopify store. To install the app on your Shopify store open this link in your browser.
https://apps.shopify.com/product-and-variant-ids. After installing the app then read below to know how to use the app.

After installing the app, you will reach to a page where you will see the list of your eShops connected with this app. Before starting you will need to sync your all products related data to this app. Go to account page and click on sync button given in the row of eShop. On sync page you will be able to sync products data on this app. To sync your data click on Synchronise button given on sync page. After your click then data will start syncing. On sync page the synced numbers will keep on blinking. You will observe syncing progress through those blinking numbers. Do not refresh page until all data get synced successfully.

Export free demo files.
After completing the sync process, then click on the ID's Exporter link from the left side menu bar. After that you will see multiple export format sections on the page. You will see demo buttons on those export format sections. Click on any demo button to export upto 100 rows in demo files free of cost.
Enter into appropriate section
You will see enter buttons on each format sections. To export full records on spreadsheets, enter into your required section. For example if you want to find & export ids for Google merchant sheet then enter into its export format section.

Export ids page-wise
After entering into appropriate section then you will be able to see list of ids along with other columns of that format. On that page you will see an export button. If you will click on that button then rows shown on that page will get exported on a CSV file.
Export all ids
On that page you will also see an export all button. If you will click on that button then all rows of that format will get exported on a CSV file.