when to use archer's signet for traps?

http://wiki.guildwars.com/wiki/Archer%27s_Signet

I want to use this skill as a trapper.

Are the conditions extended when the trap is set while the signet is active (meaning even after the signet ends, the extended condition is still applied to the trap)? Or do the conditions extend only when the trap is triggered while the signet is active?

Another way of asking is, do I set traps first, then use signet? Or signet first, then traps? Thanks.

The signet needs to be active (and you need to have your bow equipped of course) when the trap is triggered, as that is when the conditions are inflicted. Given the signet's long duration though, you should have no problem just maintaining it at all times.

I just have to say, that's actually a really interesting idea for a trapper build. Might I ask what other skills you're bringing (specifically, which traps)? I'd advise Flame Trap, since burning is the highest dps condition out of them all, and something that does bleeding (such as barbed trap). Since you're going to be relying on damage-over-time to kill off enemies, and not simple spike damage, you'll want some defense for yourself too; namely, some dodging stances to remain alive while your enemies burn to death. Whirling Defense, Dodge, and Zojun's Haste come to mind here. You could also use Tripwire (NF) so that enemies have to endure a second or so of complete helplessness on the ground in which the burn+bleed can really do damage. So we have:
Archer's Signet, Barbed Trap, Flame Trap, Dust Trap, Tripwire, Whirling Defense, Dodge, (something else).
In answer to your original question, as Toraen said, all duration modifiers in GW take effect when the related effect takes place. So if your modifier on your sword is "Enchants last 20% longer", you want that sword equipped when you cast your enchantment in order for that to take affect.

Keep in mind that the signet only increases the length of the condition(s), not the severity. You still need to kill the enemies before they kill you. I often found that it was more worthwhile to use skills that increased damage rather than condition length. (In which case the enemies were often dead before the conditions expired.)