Building a mobile ad network requires a few key steps. First, you need to understand the mobile advertising ecosystem. Research how ad networks function and what type of ads they offer. Next, develop a platform that can serve ads to mobile apps. You'll need to create software for ad targeting, tracking, and reporting. Partner with app developers to integrate your ad solution into their apps. Offer competitive pricing and flexible payment models to attract advertisers. Ensure that your platform supports various ad formats like banners, interstitials, and videos. Finally, invest in marketing your ad network to both advertisers and developers.