In order to find the DH Parameter of a robot arm with 6 DOF you should follow a set of rules:
First DH Parameter:
You have a rotation around the Z-axis and a translation along the Z-axis
You have rotation around th X-axis and a translation along the X-axis.
You could handle the Z-axis first and then the X-axis (standard DH parameter)
you could handle the X-axis first and then the Z-axis (modified DH parameter, sometimes also called DHm (or Craig))
This gives you two differnt DH transformation matrices (and less or more work afterwards)
Now the rules
Draw a kinematic model of your robot
draw the model as wire frame in the XZ-plane (why: see above) and all axes in Zero position (just to make live easier)
do not scale
the rotation of the robot axis is the z-axis needed for the DH parameter (normally here you have to check for the direction of rotation)
in most CAD/CAM systems X-axis is red, Y-Axis is green and Z-axis is blue (easy to remember XYZ RGB)
will be cont ....