Like always, it is best to clean your mini oven, like any other item actually, following the instructions in the manufacturers manual. Remember, always RTFM (Read The Freaking Manual 
)! But things can happen and you loose, or misplace, your manual, in that case follow these simple steps, adjusting them to your particular needs, and you should have a clean oven in no time.
1. Unplug your Mini Oven
This should be a no-brainer, but I still say it, always unplug electrical devices before cleaning them.
2. Put your oven in a comfortable location
Comfortable for cleaning I mean Put it on a flat horizontal, surface that can cope with a bit of water spillage and in a position that allows you to reach each part of your small oven comfortably.
3. Remove and clean accessories
Take out all trays etc and clean them separately, normally some hot, soapy water will do the trick. Be careful not to use any abrasive cleaning liquids and / or cloths. Normal oven spray should be fine, just make sure it is manufacturer approved for your model and that you use it only in well ventilated rooms. Alternatively you can put them into the dishwasher, but only if the manufacturer has marked them as “dishwasher-safe”.
4. Clean oven outside
Wipe the outside of your oven from top to bottom with a damp, soapy cloth. “Rinse” with a damp, non-soapy cloth. Pay especially attention to hidden corners where dirt can accumulate. Dry carefully and make sure that no water has entered the ventilation openings etc.
5. Clean oven inside
Use either a hot, soapy cloth or manufacturer approved oven spray. Make sure to “rinse” well with a damp cloth that has been soaked in clean water. Repeat until all soap residues and dirt have been removed. If you don’t do this well, your next pizza or bagel will have a distinctive “soapy” taste
6. Clean surface
Don’t forget to clean also the surface where your oven or grill resides normally, it is astonishing how much dirt can accumulate under a household device!
7. Put oven back together
When all is clean and dry, re-unite your oven with it accessories. Put oven back in place and let is completely “air dry” by leaving the door of your mini oven open. Ideally you should clean your oven in the evening, giving it time over night to dry out completely in case that water has entered where it shouldn’t have.
You should now have a clean mini oven, ready to prepare you something nice for breakfast. Bagel anybody?