When building my 10mm fantasy armies, I gave some consideration to how I would base them. The chosen rules ("Warband" by Pendraken Miniatures) require units to be based as a single entity on 100x50mm bases, rather than several small bases combined to make a unit as with most rules, so I was concerned at how I would base them with so many figures in the way of the glue, flock, textured paint, etc I use for such activities.
So, her for your delectation (or for you to ignore should you so choose), is my way of doing it. I make no claim to originality or even to common sense in how I achieved my aim. You may well read this and think "what a *********" (insert suitable derogatory term) at how I might have missed something obvious, but here goes.
Firstly, however, let me show you the sort of thing I am NOT concerned with here, such as this Elven command unit, seen in the first two photos. There is plenty of room in and around the figures in units like this to just stick them all down at once and texture and flock to your heart's content afterwards. So, in such cases, just get on with it!
NO!!! We are concerned with multi-figure units!
1) So, first up, gather your materials. A selection of textured paint, flock, white modelling glue/ PVA, cocktail sticks and bamboo skewers to apply the textured paint, small rocks sold as model railway ballast, grass tufts, bits of twig...you know the sort of thing, a selection of which is below.
2) Check out the two photos below, both of the same unit. Once you have your materials, take a 100x50mm base (or whatever size you are using) and paint it on both sides to help prevent warping later on.
3) Once this is dry (and after adding the little box for the unit record dice at the back left corner of the base, apply any twigs or rocks you want on the base, gluing them down securely. These will need to be bedded into the subsequent earth and grass textures to look the part.
4) Plan how your unit is to look and glue down the first row of figures, who will form the front rank of the unit when seen from the front of course.
5) Once these have stuck securely (I use superglue for this), take your chosen base texturing material (Basetex, Greenscene textured paint or whatever) and apply it around this front rank of figures, but ensure you cover enough of the base behind this front row to take the texturing up to where you intend to deploy the second row of figures. I use a cocktail stick in the main for this, but a bamboo skewer is fine for larger, less detailed work. Sprinkle in a few of the tiny rocks from your pack of ballast as you go. They will be held in place by the wet textured paint.
6) Whilst the textured paint is still wet, glue down the second row, ensuring that you butt the front of the figure bases into the texture you have already applied so that it largely covers the front of the second row figure bases.
7) As with the first row of figures, apply more textured paint around the back of the second row of figures, again leaving enough to enable you to stick down the third row, again ensuring you push them partly into the textured paint around the second row of figures so that the front of each base of the third row figures is covered. Again add a few little ballast rocks to suit.
8) Then finish off the rest of the base with textured paint. Take GREAT care filling in any gaps by dropping in moistened textured paint form the end of a cocktail stick, pushing and shoving it into accessible gaps etc. If you do not take care, you may well end up dropping it all over a figure. If you do, wash it off immediately with water! Add a few more ballast rocks to suit.
9) Once the whole base has dried, you can add grass. Mix up a dilute solution of white glue/ PVA, apply it at the end of a row of figures and simply let gravity take it down the channels and around the figures. DO NOT allow it to build up like a snowdrift around any individual figures, but tilt the base to manipulate the flow of the glue.
10) Once you have applied the glue you want to where you want it, sprinkle flock and grass in a mix that suits your taste over the glue and tip and tap off any excess onto newspaper, ready to collect it up later for re-use.
11) Once the grass has dried (give it at least a day), use whatever paint you like to highlight the bare earth of any visible textured paint.
12) Add tufts or other effects to the bases to finish off.
As you can hopefully see from the photos, a little care and effort, together with doing things in a suitable order, can lead to some fully based and useable figures with no gaps showing to spoil the overall effect you want. Job done!
I based my Goblins in exactly the same way, so it works for everything I have tried so far.