You need to do this only on the Seller Hub here:
https://www.ebay.co.uk/sh/ovw
Then find the item in Active listing, tick the little box on the left, scroll up to the Sell it Faster tab from drop down menu there, select Send Offers.
You may need a PC / laptop and not the app or mobile browser for this.
Your offers get sent to buyers who satisfy a number of conditions including:
1. They haven't received an offer from you for the same listing previously.
2. They haven't purchase the same item from you previously
3. They're watching your item
4. They've had the item in their shopping basket for a period of time
5. They've viewed your listing multiple times
6. They're a buyer in good standing on eBay.
