I only had to do it once with Spike. Now he has rocks, too, and he files his own nails.
It's just like with a cat or iguana - gently hold the foot/toes and clip straight across just in from the tip. Don't go too deep or it will bleed. If they won't hold still, you can try wrapping them in a towel to settle them and contain them.
Here's a link to iguana nail trimming on Melissa Kaplan's site with a diagram of how the nail is structured - I assume uros are similar if not identical:
trimming
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Torey
Eugene, Oregon, USA
1.1 Uromastyx Geyri (Joe and Arthur)
2.0.1 Uromastyx Dispar Maliensis (Tank, Turtle, and Spike)
1.2 Anolis Carolinensis (Bowser, Leeloo, and Sprocket)
1.1 African Dwarf Frogs (Bruce and Sheila)
1.0 Betta Splendens (Mr. Miagi)
1.1 Felis Domesticus (Roscolux and Jenny)
And several miscellaneous community fish
