To start modelling the arm, in Front view add a plane, select a vertical
row of vertices, and in Top view, Extrude and Move them like the previous
method used in the leg.
Delete some Edges, or Faces, Make new Edges, Subdivide them and Build
faces to model a upper arm protection more or less like in the image above.
Select the low row of arm vertices and Extrude down five times. Use the
same method as before to make a lower arm protection. Extrude again two
times to make the hand palm
To make the fingers, in Front view add a Cube, select his low vertices
and extrude down three times. Make four Copys of the finger, Scale and
place them like the image above right.
Start to conect the fingers and Build the hand. Select
the adjacent edges of two fingers and Build a face. Repeat this action
for the others fingers.
Here's the external side of the hand ( finished), use it as a guide. Note
that i've modelled a little hand protection.