First I glue the pieces together of course. Then I fill in some apertures or openings with the same white glue. This doesn't look too good when you don't paint them, but I do. I use white acrylic paint, the same for artists, but I use the cheapest brand. I paint them with a brush that I have bended a bit, this is easier to paint a model. I use the paint not too much thinned with water. Such acrylic paint is in itself very strong, it's like glue. I use this also to fill some imperfections. I give them at least three layers. This sounds like a lot of work, but in fact not. I paint several models at the same time, so it's band work. Plus you don't have to worry about clean gluing, the paint will cover every dirty spot. Another disadvantage though is that the model should be rather strong before painting, otherwise single sheets of paper, like for arms or so, will become weak when painting. So I give such parts a double layer of light cardboard.