evenly?
Pull the flap you have just made evenly away from the first cut, in the direction of the fibre.

Presumably with an even tension so it doesn't come away skewed?

or maybe at a flat angle.

or does it mean "flatly", that is, keeping close to the material and not up high at an angle?

I hereby give Anne(gb) one Synchronpunkt!

@wpr: I thank you, kind sir:-).

What about using peel instead of pull, which to me implies more of a sideways movement than an upwards movement, and if necessary adding "keeping close to the material"?

Too right, especially as this is about "peel ply". Thanks Anne!