OK so, as someone who paints minis, I can give you some helpful hints that might work.

the biggest things I see here are 1) incomplete coverage (I can still see a lot of black primer, especially along the right arm holding the bolter, and on his armour plate underneath the imperial insignia). Making sure you have 100 percent coverage will make the model look a lot more complete. I suggest you work 'bottom up', first painting all the lowest and hardest to reach areas and then working on the upper details. You can always smear, say, boltgun metal on the insignia while painting the chest armour, and then cut in against those mistakes with the gold you use for the insignia. Professional painters do this all the time, it prevents cutting in twice (paint the ceiling first, get paint on the walls, then cut in the wall against the white paint, so you're only doing it once and not fighting 2 cut lines). If you want 'blacklining' to show your details, I suggest a heavy black wash against details like that. It will show the detail lines quite well. Boltgun metal with a heavy (or multiple) black washes looks effing fantastic for metal. It will also help to remove those bright bright effects on your boltgun.

2) black. Black tends to suck as a colour because you can't get any really depth for it. I would mix codex grey (or any grey) and black to make a very dark grey that you can highlight with lighter grey, or shade with badab black. I use badab black wash a lot. My knarloc green and badab black wash bottles went empty way before anything else did in my paint kit. Imagine that black fabric is still going to reflect light and look 'greyer' in areas.

3) I think your skin looks good. I've never painted humie skin and it looks like a nightmare for me

4) Shaky Hands. I also have very shaky hands. Its one reason I like drawing orks so much, they have rough lines! I am currently in a huge battle with my new laptop and my tablet, I can't get the old feeling right and its curtailed my drawing atm. But this also goes into mini painting. To deal with that, let me try and describe my painting posture.
I keep both elbows on a table, or on a raised leg, or something. One hand holds the mini, the hand bending back so my wrist is exposed below the bottom of the mini. My other wrist (the hand with the brush) is balanced against that first wrist to form a whatever, fireball from street fighter pose. That way my hands are stabilized. I am often leaning back against a couch or a chair while doing this.

5) Eyes, I can't give you any help for that one, as I barely paint eyes, only red dots really. I often use a black wash to even out anything. But anything too white stands out. Try a yellow base, honestly, and then maybe a bleached bone for the white of the eye. Do the same thing with the gloves. Or use a white with a tan kinda wash (ie, get some paint really wet with water and turn it into a wash).

6) lastly check my last journal post (made a year ago!) for mixing paints. a wet palette is your best friend. Most like, you can make one out of kitchen stuff right now. I mix my paints. My ork skin formula is 1) mordian blue, knarloc green about 30/70, 2) knarloc green, 3) knarloc green with ilydrian or whatever darksun, 70/30, then the same mix about 50/50 and a delvan mud wash. I think it looks really cartoony, dirty, layered, orky and great.

on that note the tartan design is perfect. Now I can't emphasise washes enough. I know they're kinda cheeting, but if you went with a lighter black/grey pant, those colours, a lighter red and washed it all with black or delvan mud, it would look a lot sharper.

Practice on some of your guardsmen. Like my orks, you can practice and get really much better than before, and hey! 30 troops painted!

Have ya tried washes? Gives some good effects and helps a lot!

Keeping your hands still is the actual key to good painting. Without it, it can't happen. There are a number of tricks of the trade to find on the internet if you go looking for them. I sadly can't show them directly, as I have naturally very steady hands, so this has never been an issue for me. But I have come across tips and tricks online from time to time.

Ok, comment and crit on the painting:

Firstly, the pattern on his trews are very nice. I like the ice blue for some reason, it makes it looks singular and stands out without being too strange. Blue is one of those colours associated with nobility, so it works here.

But my main complaint/crit would be the thickness of the paint as you've applied it. It also is single colour, it needs shading (with Citadel Shades or home made washes) and highlighting. The red of his clothing is far too thin and the black shines through. Red is a tricky paint to apply because of this transparency in the colour, despite the paint itself being thick. The only way to deal with this is to thin it and apply many thin layers on each other. Sounds counter-intuitive, but it works.

But really, that is my main critique: too thick paint and lack of definition. The problem with unsteady hands is something only training can help. But you are still very neat and the tartan coloured trews are a very nice touch.

Keep at it!