It might be easiest to let the vet do it.  If you are inexperienced and cut too short, they may bleed...then you'd have one P.O.'d macaw while having to staunch the flow of blood from the bleeding toe nail. 

 

I used to file the nails while the bird was in the cage....I'd hold the toe and play a game so they thought it was fun.  Now....I'm trying to trim my Congo's nails through the bars but haven't had luck yet.....I'm gonna buckle down again tomorrow and give it another try.  My Timneh I'm still able to file through the bars.

 

 

Gee....I wish I had some of the birds that will just lift their foot for that trim. 

Little guys, I use the toenail clipper too through the cage bars. That includes the greys. They like having their toes tickled. The macaws.... I let the vet since both of them are rehomes, and haven't gotten used to the foot touches... we're working on it. Fortunately, I have several differnt types of perches that promote good grips, and work naturally to keep the nails in check. Angel-Rose has one toe that was broken at some point before she came, so that nail doesn't come in contact with any perches, so the vet does it. She though is very calm about it.