To do it manually, please follow the below steps:
1. Login to your Admin dashboard
2. Navigate to Catalog > Products
3. Click on 'Edit' button against whichever product you want to edit.
4. Under 'Discount' tab, select the customer group you want and fill the rest of the data- quantity, priority, price, date start and date end. Please see the screenshot here-
http://prntscr.com/m5qsy05. Click on Save.
To do in bulk, please follow the below steps:
1. Open the System Formatted Excel Sheet.
2. Goto the tab 'Discounts'.
3. Please fill the required rows- product_id, customer_group, quantity, priority, price, date_start and date_end.
4. Import the sheet in your website following the steps provide in the below URLs: