Hi, just finished other puppet while ago and start rebuild one of my character.

Now, what it's drive me crazy about is put foam head together is everytime I glue the middle head (After you glue darts and I use melonhead foam pattern)
is alway end up not straight. It alway start front head little straight then, it slowly way off the track to the back head and it not even straight
in the middle head. What did I do wrong? Also I try both hot glue and contact cement.

The Melon Head pattern should line up pretty well. Try starting at the top middle of the head and work to the front then back. Foam is flexible so just like fabric you might have to "ease" it in when gluing.  Easing is a sewing term. It means that if you have a bit too much fabric on one side of a seam you stretch the other  side just a bit when sewing to get them to end up even.

Hi Shawn, try again few days ago as you suggest starting at the top middle of the head and it still not straight. I trace pattern carefully, cut with x-acto knife perfectly and still not line up nicely. I'm getting worried as I'm not going to build many puppet if this not line up straight foam keep up. :/

Maybe try lining up both the back and front then work to the center.  I've never really had this much of an issue getting foam to line up. One tip that may help is that if you glue with contact cement then you can loosen the seam and try again by hitting it with heat from a hair dryer. Heat releases the glue bond so  you can try again.

I like the Evil Ted Method of making registry marks on your pattern. As long as you make sure that they line up as you're going your seams should remain straight and ends should meet clean.

I found a gentle had with contact cement was more successful than pressing hard on the seam that could smash it and make it crooked.  If the foam is a little longer on the ends just trim it off to be straight. I agree with Shawn that the most important part is the top seam, best to start at the top and go down the sides. Fabric will hide a few faults too, you don't have to be perfect. The best seamstresses have seam rippers for a reason, fix it if it bothers you.