A unit of Night Goblins may include a proportion of Netters. The Netters use their nets to entangle the Goblins' enemies, rendering them unable to fight.
Netter models always fight exactly like an ordinary rank-and-file Goblin of their type and are assumed to be armed and armoured in the same way too. So, in a regiment of Night Goblin Archers the Netters are assumed to have bows as well even if individual models appear solely preoccupied with waving their nets about! We can safely imagine that sufficient Night Goblins have concealed weaponry nestling beneath their robes and leave it at that (it is best not to probe too deeply beneath the robes of a Night Goblin).
Netters can attempt to entangle enemy models before the first round of a combat. Their special ability only applies once during a combat engagement, after which they are assumed to have thrown their nets away and resorted to ordinary weaponry. Once they have thrown their nets the Netter model is replaced with a normal Night Goblin for the rest of the battle. This shouldn't be a problem as you're in combat now and there are bound to be a few casualties among the Gobbos...
Immediately before normal fighting begins, each Netter Goblin can attempt to throw his net over an enemy model in base contact. Roll to hit on behalf of the Netter in the same way as when fighting close combat. If the target model is hit it is entangled. Several Netters can attempt to throw their nets over the same model assuming that they are all in base contact with their target.
Entangled models lose 1 Attack from their profile for the rest of that turn. A character model or a model with multiple Attacks might be netted several times, in which case the penalty is cumulative; -1 Attack is lost each time the model is netted. However, if the model has special attacks or attacks which are not based on the model's profile these are not affected. If the model is mounted on a creature that has its own Attacks then the plaver who has been netted can elect to surrender either a mount or rider Attack, but only loses 1 Attack from the model in total.
Once all entanglements have been worked out, combat proceeds as normal with both sides making attacks in the usual order, including the Netters. Note that this means that Netters get to fight twice in the first round of combat - once with nets and once with ordinary weapons. In second and subsequent turns of combat the Netters have no further effect and previously 'netted' enemies struggle free to fight as normal.
Netters in rear ranks may fight with spears if allowed to do so normally, but may not use their nets. In a Goblin Movement phase when the unit is not fleeing or otherwise bound by compulsory movement, any surviving Netters may be repositioned in the front rank, just as you would reposition a character model. In this way you can include more Netters in a unit than will fit in the front rank at one time and use them in a second or third combat.
When a Night Goblin unit containing Netters is shot at by missile weapons the player can remove any models as casualties - and will almost certainly not remove Netters in preference to ordinary Night Goblins. You might imagine that in this circumstance the nets of any Netters who might be struck by missiles are picked up by other Goblins around them.