A lot of trojans are like off-the-shelf kits that anyone can grab. The technical skills amount to convincing someone to run an executable (email, malicious USB keys, poorly managed advertising networks, etc) and then clicking a few buttons in a GUI to connect to them and do what you will.

For the slightly more technical, you've got frameworks like metasploit which can perform network attacks, but even that is getting much more like a kit these days. Port scan for services, fingerprint them, check database for known vulnerabilities, automatically attempt to exploit known vulnerabilities, deploy payload (the trojan).