How do you diagram He really seemed happy?

I'm guessing this is what you mean by Diagram, and hopefully it is, but in the case that it's not: please discard this.

First, you would draw a line. Since seemed can be replaced with 'is,' it's a linking verb (keep this in mind).

Now that you have a horizontal line, draw 1 slightly right of the beginning, going straight up and down. Make sure that it goes through your first line.

Second, draw a DIAGONAL line slightly left of the right side of your first line. Do not go through your first line. This line is diagonal because Seemed is a linking verb.

Your subject is HE, so it goes in the first area.

Your verb is SEEMED, so it goes in the middle.

Your direct object is HAPPY, so it goes last.

Now what do you do with Really?

Since REALLY is describing how happy he felt, it will go on a diagonal line under 'seemed.'