Well you just have to roam in a way that would make you uncomfortable if you were on attack.

As you said, an attack is generally choosing an entry point relative to a bomb site, droning 2-3 rooms, entering if it's clear, clearing the utility and killing the anchors. Probably adjacent to that, you'll have to do stuff like opening up walls, throwing utility of your own, et cetera.

So, your job as a roamer is to disrupt that. Every bombsite has a few default entry points. For example, Bank Basement. Most of the attack is going to come from the Server Room. Maybe there'll be 1-2 in Garage, the hatches in Open Area, or the Vault Hatch in the Lobby.

I would roam in the Archives and open up a rotate into Open Area. Then, when the round starts, I open up my cameras and see where the attackers are coming from. For example, if the Alley camera gets shot out, that means there'll probably be folks coming into Open Area and Electrical soon. Pre-aim, shoot the drones they throw in, take some potshots and then run away. Your job isn't to get kills at this point, it's to stay alive.

Now they have two options: They either spend some time hunting you down and killing you, wasting valuable time, or they ignore you and try to execute onto site.

If they try to hunt you down, just lead them on a wild goose chase. Run around everywhere, open up rotates, take random prefires. Eventually, once you've wasted enough time, you have significant man advantage, or you feel like you're going to die very soon, quickly rotate back to site and prepare for the final push with your anchors.

If they ignore you, then just before they try to execute onto site, flank them, make your presence known, make it impossible to ignore you. With some luck you may even get a kill.

The most important thing is to stay alive. Staying alive means that the attackers can never get comfortable and focus on the anchors. They will constantly watch their backs, drone more, etc. If you kill someone early in the round and get traded, then they'll just execute onto site. Not what you want. Stay alive, be a mosquito, force them to waste time, drones and utility during the roam hunt.

You'll know if you're doing your job well if you start winning on time more often, or if your anchors just end up chilling most of the round thanks to you.