To get started with index based price you first have to activate the function in your Organizational Settings. If you have not yet done so, follow the guide here.
1. Go to "Articles" under "Administration".
2. Choose "Price Indexes".
3. Choose "New index".
4. Add details about your index.
Add a name for your index, a description if needed, and fill in the table for index values and month. Save your index by choosing "Create".
5. Create an article that uses your new price index.
Go back to the "Article" page and choose "New article".
6. Add details about your article.
Make sure "Use price index" is red to show the relevant fields. Add base price, a date for when that price was applicable and what price index should be applied. When saved, your price index article is ready to be used in orders - as seen in the guide here!