The steps below need to be completed on a PC, the mobile version of Facebook doesn't have the same features.
On your browser, go to facebook.com/products.
Choose the store you created on your Facebook page. Tap here for steps on how to create it.
On the left side menu, tap on Data Sources.
Click on Add Products.
Select the second option, Use bulk upload.
Click on Scheduled feed. This way any changes made on the app will be reflected on the shop in Facebook.
Include your catalogue's URL adding /feed at the end and click on Next.
Without /feed, the process won't be completed.
Select the frequency you want Facebook to check for updates in your catalogue.
You will also be able to force this update from within the Overview settings in your Data Sources page.
Name your data source if you want, select the currency the prices will appear in and tap on Upload.
How long the process takes depends on how many products you have stored in the app, confirm that the amount uploaded is the same available on the catalogue.
If any errors occur, more information about them will be listed below the Result.
Congratulations, you now have to sources for your customers to access your products!