Um... ... carefully?
When I do such wings I cut the spar slots a hair deeper than they need to be. Then I push the turbulator spar strips in using a piece of balsa until they are just flush and then glue them in place. No or very little sanding needed.
If yours are sticking up then just go easy and check your progress. Use a fresh piece of sandpaper and don't push down on it. Just glide the sanding block back and forth and let the fresh sharp grains and weight of the block do the cutting while you keep a close eye on the progress.