Step 3: Work out the arms
With the exception of the shoulder and elbow armor meshes, the arm is actually split into 2 meshes. The first mesh is the upper arm (which is connected to the torso) and the second mesh is the lower arm and hand. The topology should flow around the top of the shoulder/underarm and down the arm.
Try to maintain this flow down the arm through the armor plates. Add some extra edge loops around the elbow for deformation and extrude down slightly to hide the end of the upper arm where it meets the lower arm armor.
The lower arm is separate from the upper arm. Create an edge loop beginning at the top of the lower arm plate, closest to the elbow, and continue extruding down towards the wrist. You may need to merge some vertices if the topology gets too crowded.
Step 4: Hands
Create a cylinder and delete the caps from both ends. Roughly scale and position the cylinder to match the index finger. With the ZBrush hand mesh selected as the Transform Constraints > Other Surface in the Modeling Toolkit, move the vertices of the cylinder to snap them to the ZBrush mesh. Add edge loops around the knuckles to allow for deformation.
Next, duplicate the finger mesh, and position and scale into place for the remaining fingers and thumb. Adjust the vertices for each finger and then combine (Mesh > Combine) the fingers into one mesh.
Extrude the edge loops down the hand; again, you may need to merge some vertices where the topology gets too crowded - however, try to keep the edge loops running from the fingers down the hand.