Yes, I can take apart pens, reduced the amount of Imitrex to 4mg, put it back together and take the SubQ injection in seconds while in the dark rocking & rolling with The Beast !! (But I use DHE45 IM injections up to twice daily (PRN) in hopes of preventing any reason for needing an Imitrex injection for a cluster attack)

6 mg of Imitrex is usually too much for me, 4mg works fine.

Detailed instruction from me ??  .... I doubt that. I'm not qualified. Also,  I don't have the same pen as you do. But according to your pictures (Which were great), It looks pretty basically the same thing. Getting the 'pen' itself open to be able to get to the glass medicine container (the syringe) and then using a Q-tip or similar object to push out (X) amount of medication. Simple enough !??! I GOT IT ! You can keep what you don't use in a Sterile vial (pretty cheap) and draw it up at some late time and at any different amount you may require at that time.

Good Luck!

I4B

Disclaimer: I" NOT A DOCTOR and in my opinion you should seek a doctor for questions, comments, concerns, etc.  *Sorry, had to include that nowadays.
  Ok, carry on !