There are two ways to insulate triangular attics behind kneewalls.
How to insulate a single story attic.
This method can be made to work but the necessary air sealing details are demanding and fussy.
To build a catwalk you would usually elevate a section of the framing perhaps with 2x6s installed at 90 degrees to the joists and you would install osb or plywood on top of the elevated framing.
A better approach is to insulate the roof slope above the attic.
Press and tuck the edges of each part of the blanket to make sure they fit snugly and that there are no gaps between.
If you need regular access to your attic it makes sense to build a catwalk before you insulate the floor of your attic.
Lay a two by four on top temporarily compressing the insulation to a manageable.
While wearing dust mask and gloves carry r 38 batts of unfaced fiberglass insulation into the attic.
Lay the batts across the ceiling joists covering the existing insulation.
To cut blanket style insulation place the product over a piece of plywood with its paper or foil side down.