Good god, man! You actually did all the black with a pen? That's dedication lol. Try investing in a sharpie or another black marker. This will give you the black you want with a lot less effort. Sharpie, Copic, and Prisma all make good ones. Next, Use a ruler or other straight surface to get straight lines. Sketch where you want then with pencil and ink them with a ruler. Again, less time taken to complete so you can make more art! Lastly, look into crosshatching, gradation or value scale, and rendering. Try playing with how much pressure you put on the paper with the pen too. You'd be surprised at what you can create with just a pen. Those are just a few tips that helped me along the way. I've been doing art for 30 years and I still continue to learn. Great piece!

Well your way better at art than me

Well you definitly have an understanding of the existing asset, but I would recommend adding your own flare to it. Add some variation to the ink, make some of it darker or lighter, highlight the different parts of the skull. You have the ability and the talent, but you need to challenge them. Keep up the good work. I look forward to seeing you progress