It's easy enough. You need a Y-cable adapter to feed either the left or right channel feed of your source to both right and left amp RCA inputs at the same time.
Then a short jumper wire between the negative of one speaker output jack to the positive of the other speaker output jack.
Then attach your speaker to the remaining free speaker output jacks.
Make absolutely sure your amp is capable of bridging (what this connection is) or your results may result in smoke.

I currently have two SE84UFO2 bridged mono. The primary improvement is increased volume.
Since I bought the second UFO2 about 1.5 years after the first UFO2, the tubes on my first UFO have more burn-in. So I noticed a difference in the sound from the left and right channels. If I had thought about it I would have had Steve send a full set of matched tubes for both amps. This may have eliminated the difference in sound.

Don't get me wrong, the difference is sound is minor (to my untrained ears) and the new amp sounds great!

I have to say that my experience parallels Rivieraranch's. I have a 10W 300B SET amp that I just love the sound of, but thought I needed more power. I've become friends with the owner/manufacturer, who lives in the area, and he brought up a second identical amp to experiment with. He wired them internally as monoblocks, and they did get louder, but they lost that "magic", which wasn't acceptable to me. I have a number of other amps with more power, but none that sound like that. So I'd say that if you're happy with the quality of the sound that you are getting, but just looking for an increase in volume, go with a more efficient speaker. Maybe Randy's Betsy open baffles, which are WAY up there on the "bang for the buck" scale.