The first step is to click Manage at the bottom left corner of SalesScreen. 

A menu will pop up, where you can proceed to click on Targets under the Company headline.
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You will now be presented with a table showing targets for you current period. If you haven't created any periods yet, have a look at how to create periods.

Click Add targets in the upper right corner.

You willl be able to select one or more periods, a product type and a product before you add your target values in the number input fields. 

Note that if you do not specify a product in the product dropdown, your target will be specified for the product type.
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Example: Let's say you add targets for Sales and don't specify a product. Your target will account for all products for that product type. 

What you need to consider here, though, is that you can't set a target on a specific product and the whole product type in the same period. Choose between them.

Click Add targets and your targets will be saved.