How do I measure/sew a plain crewneck collar?

[zebaby]

I'm making a top for my 10 mo DS out of fleece. I'd like to make a plain crewneck collar but have no idea how to make it fit over his head without making the neckline too big. The top is basically done except for the collar - I have a hole but it's definitely not big enough.
Here's a pic of it unfinished: http://s153.photobucket.com/albums/s...ean1007072.jpg
I planned on finishing with some bias tape to cover the cut edge. Should I just cut a slit down the back and finish with some more bias tape or is there a way to measure so that I can leave it like a normal crewneck?

[orangefoot]

That looks really great! I love the colours and the shape.

Fleece is quite stretchy so you should be able to cut the hole large enough to stretch over his head. Then you could add ribbing to the raw edge to hold it in but still let it stretch. If you add a woven tape you will lose the stetch and recovery of the fleece and the neckhole will be stiff.

Here's a photo tutorial of how to do it. http://www.burdastyle.com/howto/show/111You can buy new ribbing or recycle some from an old sweatshirt.