The sporran is not what causes the apron to fall between your legs when sitting you use your hand to do that.

Many new people wear their sporran too low. It should not bang your legs or swing around wildly when walking. It should also not cause your aprons to bunch up under the sporran. If you find you need to smooth your apron after a few minutes of moving, your sporran is probably too low. A good rule of thumb is;
With the kilt at normal, traditional height, above the naval, your sporran should hang just under your naval and just above your genitals.

I suggest you take a look at Hamish's photo albums. Anytime I have a question about the proper and/or appropriate wear of the kilt I look at how Hamish does it first. Then I go from there.

You can also look at Hamish's video on sweeping the pleats and sitting which is a sticky post here.