Hi and welcome to the Planet!
For eyes my trick is to paint the whole slit in a black, darkbrown or darkblue colour. Then add a little bit of the whites(a mix with about 1/3 flesh and 2/3 white) in one corner or both. Just a tad bit as the white part of an eye usually is no more of the area than 1/3.
Free hand heraldry is hard. But I have done ok in the past and I cant even draw a straight line with a ruler. I use Humbrol but make sure that the base coat is really dry, then when painting the pattern I can trim, or erase mistakes by whiping it away with the brush dipped in solvent. Touch up with the basecolour, and you do all this back and forth until you have the result you like. If you like me, dont have natural talent it will take some time but in the end get the job done.
You can also sketch the intended pattern with a coloured pencil before to get an idea of the layout.
Hope this helps.
Cheers
Janne Nilsson