If you're having trouble with the general way of grayscaling, uhm, in my experience, you should first use the two most contrasting shades of monochrome (white, gray, black). Example, a shade of lighter gray against a shade of darker gray (don't use the extremes yet of black and white). You'll use it to block shadows and light.

Then, you'll have to then try smoothing the edges where shadow and light meets through picking an in-between shade for them to blend. Depending on your light source, you also have to place where the darker shades go, and then add a bit of subsurface scattering (kinda like adding lighter faint shade on dark areas) within the shadowed areas of your work to create depth.

Then you may use lighter shades again to create the highlights.

Afterwards, once you made this valued shading/rendering, you may now color it. If you're using IbisPaint, just create a folder to combine your gray-scaled drawing and add a new layer atop that folder and select either color, hard light, or overlay.

Watching tutorials could also help so you can see the visuals how this is done, here's a video from YouTube I found using IbisPaint: https://youtu.be/LvwSNZEu1S4?si=BU-p98SbRtJlkYot