Step 4: Add the fill tube and cement.
Mix up a tiny batch of cement -- about 1 cup. I'm no expert at mixing it but I know it shouldn't be like wet sand and it shouldn't be like soup. If it's too dry then it won't seal and if it's too wet then it will crumble once it's set up. If you're not familiar, try a batch and see how it works.
Anyway, insert the fill tube along the edge of the sand. Stick it into the sand just a small amount (maybe 1/10" or so) -- just enough that the cement won't flow around and seal it.
Add the cement around the edge and try to seal around the fill tube as best you can. If you want to make it look less industrial, you can add decorations into the cement -- once it cures you can't.
Let the cement cure overnight.