Hi Marc, these trousers were almost certain not to be a very clean white so I would first start from that idea, the base should be a cream or buff so if you work over an acrylic basecoat use somethig like this, not straight white.
One of the general methods for shading white is just to use a mix of black and an earth colour to shade them; black alone almost always look wrong, it makes greys that are too blue, but using umber helps counteract this. You might find that the best results are with mostly brown and only a small dot of black or blue (Ultramarine for example). You can use Burnt Umber or Raw Umber for this, but red earths also work okay although you would use less of them.
I don't know what paints you have but Titanium White would be the best white to use in oils as a starting point because it is the most opaque, if you are using another white that will usually make the mixtures a little transparent which is harder to paint with in general.
Try this:
Highlight: Titanium White and a little Burnt Umber.
Shadow: Titanium White, Burnt Umber and a little black or blue.
Midtone: same colours as for the shadow but with more white and more umber.
If you want to highlight further you can just add a little more white to the highlight mix, possibly with a very small addition of ochre.
Or this:
Highlight: Titanium White and a little Raw Umber.
Shadow: Titanium White, Raw Umber and a little black or blue.
Midtone: same colours as for the shadow but with more white and more umber.
Or this:
Highlight: Titanium White with a little of the midtone mix.
Shadow: Titanium White, a little black or blue and just a touch of Venetian Red (add carefully).
Midtone: add white to the shadow mix.
Einion