How to crop for exact print sizes in Aperture 3

I had quite a few pictures printed out that that parts of the image cut off and I was told it was my fault because my crops were to large. I am new to Aperture 3 and photo editing in general.

Could someone help me out? What exactly do I need to do to export a few 8x10 and 11x14 images to ensure I do not keep wasting my money?

I have looked online and in my Aperture book and they do not specifically say.

Thank you

spice weasel said:

Bring up the crop tool and you will be able to select to either not constrain the crop or constrain it according to standard photo sizes or a custom proportion.

Click to expand...

Ok, but when i go to print the picture to fit in an 8x10, half the image is cut off..How do I fix that?