Hello.
If you want to provide your customers 10% off promo code on all products, you can create it this way:
1) Go to Promotions -> Shopping Cart Price Rules
2) Apply rule information settings - rule name, active status, customer groups, coupon (or generate it at 5th step) and validity date.
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3) In Actions tab select Apply: Percent of product price discount and Discount Amount: 10
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4) Save and continue edit
5) Additionally, if you at the first step has not entered the code, you can generate it in 'Manage coupon codes' tab
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After that client will be able to apply the code at checkout step -
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Here are also examples of other most popular promotions that can be created with the help of price rules -
https://www.mag-manager.com/useful-articles/about-magento-shopping-cart/examples-magento-promotions-...
Hope this helps.