Come on Amanda, don't overthink this. I've showed you where the "Size and Resolution" setting is for your crop tool. That's the ONLY thing you need to know. NEVER go to the Image Size dialog, ever.
Just choose your crop tool, enter the desired height and width, and the resolution that your lab requires (250 in your case). Then crop. That's all.
Then sharpen for print, save the jpeg, and send it off.
More info here.
Change your life here.
Oh, also, this one is vitally important.