Hi Mats
This is a cool reference and to be honest I thought at first it was maybe a bit too ambitious for you at the stage you are at, however from what I see from your progress so far, my first thoughts were wrong.
I would love to see you succeed with this, and I believe you can, but I'm afraid it just isn't going to happen on your "regular sketch paper", please understand that what I say now is to save you some bitter disappointment but please stop right there, your experience is not far enough along yet for you to be able to rectify any mess you make on that paper, and despite all the good advice you get here scratching and erasing have little chance of success on this medium.
So here is what I would like you to do, and believe me you will thank me for it (eventually), as I said STOP, then get yourself some appropriate airbrush paper or poster board which can take the trauma of scratching and erasing, you will find that even with little experience the task will be a lot easier as well as good for your soul, you owe it to yourself to use the proper stuff.
I just recently bought some Schoelershammer paper (there are other brands) and I can't believe how easy it is to work on, you can get it in paper form or in board from, I have the paper and the only problem I noticed was that just like your sketch paper, it starts to warp immediately you start to spray on it, however, I have found a solution to that, I bought some cheap spray adhesive and got some compressed cardboard which is normally used under carpets and flooring, cut this to the same size as the paper and sprayed it with the glue, then paced the paper onto that tuned it face down on a glass table and rolled it out with a wooden roller, a normal kitchen rolling pin is ideal for this, and lo and behold you have airbrush board at a fraction of the cost.
I would not have bothered with any of this if I didn't already see you do have some skills, so please do start again on proper paper and just repeat everything you've done until now, you will amaze yourself