"But these have to be adjusted .. the OAT must be recorded at the
various altitudes and used to adjust the pressure altitudes to
SL/standard atmosphere. Can anyone fill me in on how to do that?"

Why? You're not concerned with absolute (True) altitude; just a delta
between contiguous points. As long as you're flying through a sharp
frontal boundary during your measurements, I can't see how this will
matter. It's been a while since I actually did these manually, but Rho
(density) cancels out in a ratio of Lift to Drag, no?

L = 1/2RhoV2 CL S
D = 1/2 Rho V2 CD S

In any case, there are a number of places to get the gas law equations
to convert to ISA. Google on ISA conversion formula or somesuch - I
got several.

P3

D=