Here is the formula I have made up for students. Its been awhile since anyone asked me to mix some up, but it seemed to work fine.
From Elementary Photographic Chemistry (Eastman Kodak Company), 1941:
Formula T-12 -- Blue toner for paper
In separate containers, dissolve the below separately in about 250ml water each:
Ferric Ammonium Citrate -- 4.0 grams
Oxalic Acid -- 4.0 grams
Potassium Ferricyanide -- 4.0 grams
Filter each before mixing together (actually I never do)
Mix the three together.
Add water to make 1 liter
Immerse well washed print for 10 to 15 minutes until desired tone is achieved.
Wash until highlights clear.
There is another toner formula for slides and films (T-11) -- with instructions for making 120 gallons! For the motion picture industry, I guess!