If you have Shopify Plus you can use the Launchpad App to schedule sales on specific collections. If you're working on any other plan though, the way I would typically set up a sale would be to create a collection called "Sale" that imports products automatically that have a "sale" tag added. Then I'd export my products to a CSV that I want included in the sale, add ", sale" to the tags column, copy my variant prices over to to the variant compare at price column, then do a calculation in the price field to give you the percentage off. Say you wanted to give 20% off, I'd make the variant price column:
= CompPrice * .8
then copy that formula down the column and format the number.
When I wanted to start the sale I'd upload that CSV and overwrite the products, then when that sale was over I'd reupload the original csv and overwrite the products again.