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You can't, really, without starting cutting away on the outside, which exposes the (usually) much worse inside.
If it wants doing, you could be looking at £400 per side to do properly. Also check the jacking points under the cills, inaccurate use of the jack leads to cill edges being crushed and resultant bad rust.
If I'm being honest with you, which I try to be, older classics are a money pit, leave well alone...and I have one!
Best bet mate pop the boot pull the seal holding in the side panels undo a few little plastic clips holding in each side in place , fold in both sides you'll see 2 trenches probably full of water this is the main starting place of all the rust . This will give you a good idea of what's involved and how bad it is . Hope this helps
Those pics already show you everything you need to know at least as far as the arches are concerned...
By the time the bubbles are visible both sides are already gone...there is no "easy fix" - you will need to cut and replace with fresh metal...budget this into the purchase price.
ETA: no reasonable Impreza owner would be annoyed by a prospective buyer checking thoroughly for rust.