Righto, assuming you’re only recording one mic at a time to begin with.
Start with the individual mixer channel set to 0 (i.e. 12 o’clock), main mix fader also at 0. You can leave the interface input levels right down while you sort out the mixer levels.
Normally you’d start with the gain turned right down, but since you’ve already had it turned up you may as well set it to 12 o’clock to begin with.
Make the sound you’re planning on recording and look at the metering near the main mix fader. As aldovino already said, you want it to be starting to hit the orange.
If you’re barely getting to the second green on the meters, then you can up the gain a bit. Alternatively if you’ve got a solid orange brushing up against the red, or the peak light is flashing on the channel, then you need to reduce the gain.
If you want to record with both mics at the same time, get levels right for each mic (as above) and then reduce both mixer channel levels (but leave the gain knob).
Once you’ve got a good level coming out of the mixer, then you can adjust the input level of the interface to match.
If you’re finding that the recording is too noisy, then try plugging the mic directly into the interface. Not sure what the xenyx mixers are like these days, but I remember them being a bit too noisy for my liking.
Something else to keep in mind is that these are both dynamic microphones and as such are best suited to loud sounds i.e. guitar amps, percussion, rock vocals etc. Since they rely on sound pressure to move the magnet inside the coil, they don’t work so well for quieter sounds.
