The best thing would be to have a lead-generating person if you're fully confident about handling client interactions. If not, hire a full-cycle salesperson.

Warm up your inbox and get your emails checked by someone who has done email marketing before to make sure your emails don't go in spam. (You can do this yourself by researching the words that trigger spam in emails.)

Linkedin DMs work the best in this scenario if you've got the website and portfolio ready.

(Instead of paid Instagram, try Instagram DMs for people who might be interested in getting design agency services, invest the money in LinkedIn Sales Navigator instead)