1) First you need to get a command block.

2) Copy and paste the command below in the the Command_Block


The command: summon ArmorStand ~ ~ ~ {Invisible:1b,ShowArms:1b,Rotation:[91f],Pose:{Head:[14f,22f,0f],RightArm:[0f,351f,99f]}}

Edit the rotations as needed to get the armor stand is the right postion you want. If it helps, delete the "Invisible:" part of the command.
Also if you want to use the left arm and not the right, just simply change the name.