Let's say I need to create a 6.75"x 8.25" image with an additional 1/8" bleed all around. I could do a bunch of addition but that's a pain in the neck. So instead, I'm going to create a document (File -> New) Make this new document the size of the final art. Don't worry about adding the bleed yet.
Next, make sure your rulers are visible (View -> Rulers) and snap to document bounds is turned on (View -> Snap to -> Document Bounds) Click on the top ruler and drag down to create a new guideline. Drop that guideline to the top of your document. If you have Snap to Document Bounds set, the guideline will pop itself perfectly onto the edge of the document. Then click on the ruler again and drag a guideline to the bottom of your document.
Next click on the ruler to the left and drag a guideline over to the left edge of the document. Again, it should snap itself to the edge of the document. And, of course, we need to click and drag once more from the left-hand ruler to add a guideline to the right-hand side of the document. So now you should have blue guidelines on each edge of your document.
Now all you need to do is open the Canvas Size windows (Image -> Canvas Size.) Make sure the "Relative" check box is checked. and that the Anchor Point is in the middle. Then set the width to twice the bleed amount. Remember you need to add bleed to both left and right so this value needs to the be equal to bleed amount x 2. So in our case 1/8" x 2 = .25" Then also set the height to twice the bleed amount which would also be .25" Then hit OK.
And voila! You have your document set up with the proper amount of bleed and all of your crop lines marked.