So, I'll admit this was me. 100%. So, I'm currently in Commercial MEL training at a 141 school, flying Barons. I'm at the point of my End of Course checkride and I just happen to get the check pilot who is a notorious hardass. He's also the only check pilot who is current for multi-engine so there's that. It really wasn't my best day from the start. Nerves had me puking my guts out and I couldn't keep anything down. (Side Lesson, know when to call the flight off) Takeoff and climbout was fine, we do our cross country calculations then we get into single engine ops. Power up, clean up, identify, verify, diagnose, feather, secure. No worries. Fly for a few seconds on the feathered engine then he gets me to restart it because it's like -10C up there and we were trying to save a bit of time. Take out the checklist and start going through it. Then something happened. I don't know what but I got thrown off the checklist. I go through it what I thought was completely, but forgot the magnetos. We were flying with an unfeathered engine, thinking it had started and was warming up, but minutes passed. We thought a lead on the CHT had been knocked loose, as these airplanes were prone to doing thanks to rough handling students, and turned back to the airport. Only then did he realize that I had forgot the magnetos and pointed it out. Engine fired up right away and I ended up calling off the rest of the flight and going back with my tail between my legs. Use. Those. Checklists. I had created a potential emergency situation by forgetting those magnetos, all because I didn't slow down and properly use the checklist. It reminds me of this instructor I had years ago that told me to slow down and actually read the regs. Of course you have the generalities down, but when you're looking for specifics, you can't just skim.