Constructing a binary tree with given traversals
You are doing well. From the postorder determine the root, from the inorder determine which letters are in the left and right subtrees.
Once you have done that, you can proceed in the same way for the subtrees.

In the example you start with postorder DCBGFE A
and inorder BDC A FGE.

Using postorder you determine the root as A, and using inorder split the letters into left BDC and FGE. 
Thus (1) the left subtree has postorder DCB and inorder BDC
while (2) the right subtree has 
postorder GFE and inorder FGE.

Let us look at left subtree (1) which you mention in your question. The root is B, and both DC follow to B in the inorder and thus are right from B.
How to place them. This tiny subtree has postorder DC and inorder DC. Hence its root is C, and D is before C in the inorder, and is in the left subtree of C.