Filling holes in floorboards

[dllive]

Hi
The carpet fitter said I need to fill the holes in the floorboards otherwise the carpet will sag in the holes over time.
How would I go about doing this? These are the examples of the holes:

Ive no idea what Im doing and have no tools/materials. What I do have is some laminate and some PVC glue. Could I get away with cutting some laminate to size and glue to the floorboards?

 

[foxhole]

Sheet it over with hardboard .

 

[cdbe]

Cheap "no nails" from Toolstation. Build up in a few layers as I find it doesn't set so well in a thick dollop. For any large holes cut some bits of your laminate slightly bigger than the holes, drill a hole, thread a knotted string through, feed into the holes and pull up against the underside of your boards to form a base for your "glue" while it sets.