Hi!
You don't say whether you intend using pulp or layering.  I'll guess the latter.  REALLY smooth?  You mean glass like?  Not easy.  I'll give you some tips on improving the smoothness though.
Presumably you are moulding over something (a ball?).  Give it a smear of Vaseline then cover it with a skin of clingfilm with as few wrinkles as possible.  The Vaseline is to hold the clingfilm in place.
Use a paste mix (about 70% wallpaper and 30% PVA) consistency thick cream.  Use bond (office) paper.  Tear off the straight edges and throw them away.  Dip the paper in water;  squeeze the water out - this breaks down the rigidity of the paper.  Tear it as you work into pieces that will lay on the surface without creasing.  Smear each piece with paste and apply it, smoothing it down with your finger. 
Do one complete layer, smooth it again and allow to dry.  Give it a light sanding.  Apply another layer in the same way.
When absolutely dry, mark a circumference for a cutting line and a couple of cross lines to help you line up the two half shells.  Cut through with a sharp knife at a shallow angle.
Put the two halves together quickly (before they warp) and join with a few tabs of masking tape, then layer again.
Last layer, use kraft (brown) paper from envelopes or paper bags.  Treat it in the same way.
If the result is not as smooth as you want, you are needing gesso, otherwise just paint it or apply collage etc.
Good layering

Spackle works wonders as far as smoothing.  I just hate so much sanding.  There is a little trick I do after I have spackled and it has started to dry (about 5 - 10 minutes after applying the spackle).  Dampen your hands and smooth all the surfaces with a light touch using mainly your palms for wider surfaces.  DO NOT OVER-DAMPEN YOUR HANDS or the spackle may crack.  The results are amazingly smooth that require very little sanding.  I use this technique to easily achieve porcelain-like textures.~tammy

Once you've got the surface as smooth as you can, try using the very finest wire wool and then, lastly, the very very finest "sand" paper - this is just brown paper! A thorough rub with the reverse of sandpaperfor instance, gives gesso a porcelain finish.