It's esp hard cuz our little ones have those darn blk nails!!
Here's a tip I learned from grooming school: trim little by little until you see the black dot in the middle.
Now, this works if your yorkie stays still enough for you, but my Momo is a major wiggler. He acts like I'm going to kill him or something! In that case, I just take a huge deep breath & hope that I don't cut his quick. I just cut the part where it starts to curve. I know I can do this to him cuz his quick is kinda far back, but some dogs have their quick long.
In that case, it's ok if you cut the quick, don't panick (cuz they can pick up on your energy) just apply quick stop (or flour if not available) & some pressure on it to stop the bleeding. The quick will retract & be furthur away from the tip of the nail. Try to avoid cutting the quick tho...they remember the pain & may not like that particular nail to be cut in the future. In that case, after cutting the nail, praise him & give him his favorite treat to associate that cutting nails is not too bad.