I'll reinforce that you don't want to put more than one sheet in at a time for maximum control, unless you're toning for permanence with more dilute toner, in which case Martin describes well how to immerse the prints back to back for even results.
How I do this:
Four trays, three with water, one with selenium toner - in the sequence: water, selenium, water, water.
I keep all the prints I want to tone in the first water bath.
Then, one at a time with only one print in the tray at any given time, I lower the first print into the tray with selenium. I usually use it at 1+4 at about 80*F to obtain a color shift.
I continuously agitate the tray for an even result, making sure the entire print is immersed.
When the print is done I move it to the next tray of water, and as I do so I take the next print in the pile and put it in the selenium.
I am careful to only use my right hand (with a nitrile glove on it) to move prints from the selenium bath, and only my left hand to move anything from water to another bath.
About 40s into the toning of the second sheet I move the already toned print into the fourth tray, which becomes a holding bath.
This leaves the third tray empty and ready for the next print, and then I rotate them like this until all the prints are toned to my liking.
Then I move the prints, one by one, back to the original tray in which they were originally held.
Finally I give the prints a good washing of about an hour in a print washer.
Hang or lay on screens to dry after squeegee.
I hope that helps.
- Thomas