Hi there!
Welcome to "The Planet"!I'm using acrylics and I think I could help here.
First of all,in order to learn how to use acrylics you could read an article by Mario Fuentes that exists on "Vallejo Acrylicos" webpage.(Give it a search in Google...).
No surprise painting eyes on the figure seems difficult to you,since till now you were using oils.When painting face with oils you have to paint the eyes whn the face is done,which is very difficult and needs to be very familliar with micro-painting,so that the result is realistic.With acrylics you have the advantage to paint eyes in the begining.I always paint the eyes first,so that,in case that I fail,I don't have to remove the paint from an already painted figure.
For the eyes,this is my technique:First paint the hole eyeball in black.Don't be afraid or too careful about this step,you may fix it later(when painting the face...).
Then paint the eyeball in a white-like color(not clear white,add some flesh and brown...).You are suposed to let a thin black line that rounds the eyeball.If you make any mistakes while applying the white-like coat,fix it NOW with black(form a white eyeball surrounded by a thin black line.
Time to paint the iris(colored part of the eye).From now on you have to be very careful!Make a dark mix of the eye color(For example dark blue...) and make a point almost at the middle each eyeball(this depends on were the character is supposed to look at...just watch your own eyes in the mirror...).
Highlighting the iris:Make a lighter tone mix of eye color(light blue...) and make a smaller round point in the center of the first point.Watch out!The second point sould not fully cover the first!
Highlighting more
This may be too much for you,so you could skip it,but try to practise it as you get used with painting eyes!).make a VERY small third point in the second point,not in the middle,but near to the edge(it depend on the direction of light,this last highlight imitates the lights reflection!).This should be in even lighter color tone,but not white!!!!
Last step:Here you have too options:
1-Make a VERY small point in the MIDDLE of the SECOND point.This last point should be in black.
2-Make a VERY small point in the MIDDLE of the SECOND point.This last point should be in white.
I would like to remind you that you could remove the paint from painted figure letting them in Nitro-Cellular Diluter for two days or more.Remove Cyanoacrylic glue and Epoxy clay letting the figure in Aceton for one about one day.This way you could repaint older already ainted figures you are dissappointed in.
I would sujest a 70mm figure,but you already did.Macbeath is a cool figure,but better try a figure with fully uncovered face,so that you could practise better on the eyes.
54mm is a good scale,too.Don't be afraid,they are big enough to paint beautiful eyes!I feel proud to have painted successfully eyes in 25mm figure,so i can asure you that it's not impossible!
That's all,I hope I helped!
Good luck and the most important
on't stop trying(again...).Ask what you need to know and try again and again.Noone is born great figure painter!
Cheerz!
-Kostas.