Although painting 6mm figures is both quick and easy, I cannot say the same for doing the bases, especially if I am using the smaller Heroics and Ross figures where I mount 20 HI on a 20mm square stand.
The difficulty is that there is not much space between the strips and in applying the basing material there is a danger that some of it ends up on your beautifully painted figures.
The trick I have found is to glue the first strip, or rank of figures, on the base, fill in behind them and then while the basing material is still wet, glue the next one in place behind.
As the figures are so small I use very thin magnetic bases without any addition of cardboard. I simply stick plain paper on the adhesive side and paint it with Coat d'Arms goblin green. The magnetic base is important for three reasons: it does not warp even though it is thin; It allows you to easily use movement trays (see below); and it means you can store your figures in metal lined boxes without the danger of them falling all over each other and ruining them.
After gluing the first rank into place I apply a water based wood filler (thinned down) behind the figures. To do this I use a combination of a tooth pick and an old, thin, paintbrush.
While the filler is still wet and malleable I glue the second rank into place, pushing the figures up against the filler so that the space between the ranks is reasonably well filled out
When the glue has dried I fill out the rest of the base with the wood filler and then apply dabs of texture with a thin old paint brush. Here I have used a mix of fine sand (from Games Workshop) with white glue, thinned with quite a bit of water, and green or sand coloured paint.
You can also use commercial material like Basetex. I have added some Basetex to my green mixture as it adds variety to the texture created by the sand.
The next step is to give the entire base a thin wash of my ubiquitous Raw Umber. When that has dried I dry brush the textured areas to bring out the highlights. On the green a final dry brush of yellow really does the trick.
As 6 mm figures can sometimes be hard to identify on the table, particularly when they are not uniform and viewed from behind, I often like to paint the rear of the bases with an identifying colour. In this case white as they have white shields.
Finally I apply some flocking or static grass using white glue. With 6mm figures you have to be careful not to add too much of this as otherwise your figures could look like they are wading through waist high grass!
I mount my 6mm figures on 20mm square stands. Some people prefer to use the same sized stand as they would for 15mm (or even 25mm) and simply mount more figures on it. This does look impressive en-masse, but for me it looses some of the flexibility the smaller scale gives you, not to mention the benefits of square bases when moving from line to column and vice versa.
The beauty of small figures is that you get a real feeling of mass and your armies really look like armies, rather than a few units. In order to avoid having to pick up each stand individually, each time you move, I highly recommend using movement trays. Since the figures are based on magnetic stands, this is very easy to do by simply cutting strips of metal sheets (available from magnetic displays) to the unit size. I prefer to use the plain white sheets and spray pain them with the appropriate colour. Although you can buy green or sand versions, they are more expensive and the colour is glossy and does not, in the end, look as good as a movement tray you have sprayed with an appropriate flat colour (or mix of colours).
In the photograph to the right you can see the mass effect of 6mm figures and how the units have been mounted on magnetic movement trays. You can also easily imagine just how fiddly it would be to move each stand individually.
Under Legio VI and Civitates Bellatorum, the rules I use for my classical period ancients, a whole legion is treated as a single unit. This means that your movement tray needs to have spaces between the lines. In representing the quincunx formation of the manipular legion I also space the stands apart in each line. This gives great scope for an imaginative movement tray, filling out the spaces with texture, flocking and a few supernumerary figures. The way I have done this can be seen in the photographs below, incorporating the units painted and based in this tutorial into two legions operating together.
The empty movement trays for two legions
The two Legions deployed in quincunx formation. The Triarii have less figures per base, representing 'under-strength' spearmen as they had 600 men in the third line compared to the 1200 each of the Hastati and Principes.