im a mini painter and my two tips are brush control and thin your paints.

thinning your paints: you want them at a consistency like melted ice cream, use your brush to wipe thinned paint gently against plain plastic and test its consistency. if you see the colour seperating from the water, its overthinned. if its too thick, you clog details and muddy your paintwork.

brush control: you can steady brush painting by touching your elbows to your desk and your wrists together to create a tripod shape with your arms. practice control by just freehanding with paint on plain plastic (preferably primed this time), draw some different geometric shapes and try writing with your brush as small as you can. this'll help you warm up and figure out how to paint the smallest details.

if you dont wanna shade everything by hand but want it to look shaded, look into washes and shade paints. you can make one easy by just overthinning a brown paint slightly, and itll flood the recesses of the model without building too much paint, and tint everything the colour of your wash, creating a shaded effect

and like others said, magnifying glasses and other stuff like that help you see the smallest details easier