Scanning, Searching, Gathering targets and then infect them with your weapon;
What I mean scanning, searching? Using tools like nmap, masscan etc;
What I mean targets? IP range/subnet+ports sockets - ranges +get educated about ports +You need to make sure what device is it (Also many options/tools and recognizers for example with the same nmap for service and version);
What I mean gathering? Sort all your positive and filtered results in .txt;
What I mean weapon? Your C2 panel/arsenal, malware or technique(exploiting) to own the target and then chain them with your botnet;
The simplest way that comes to mind... But you have to understand that behind any of that tip is a whole instruction book that needs to be read and understood to work with it +dozen scenarios;
There are several ways to obtain targets, this is the most common for massive blind search. The other option that I know is "put cheese in a mouse trap" - pay or make your own traffic to your malware;
This is the way how I got my first VPS many years ago;
UPD: I never used botnet but you can try this technique to get targets;