I took the liberty of drafting you a pattern -
As I have not tested it out, I do not know if it will work exactly how you want it, but if you make it and want to PM me I can help adjust the pattern. This is basically how you do it, with the triangle bits sewn together as a dart (thus making it more 3D) before sewing the pieces to each other. About.com has a good article on sewing darts if this is confusing. http://sewing.about.com/od/techniques/ss/sewingdarts.htm
To use the pattern, print it out a little bigger than the size you want the plushie to be, and cut around the pieces leaving 1.5cm seam allowance, 2 of each for head and body (if you are using fur fabric remember which way up to have it - I know this sounds obvious, but when I started out I totally forgot and had the fur going the wrong way). Sew the head pieces to each other right sides together, leaving an opening on the neck, turn out and stuff. Same for the body. To sew them together, turn the fabric of the head inside by about a centimetre or two, put the neck inside and sew together. A Whipstitch or Slip Stitch are good stitches to use and there are plenty of useful tutorials on these online, just google the terms.
Phew! I hope that helps and if you have any questions, ask away!