In order to do a signature move, you need to use the B button when the enemy has several circles spinning around his head. There are a few ways to make this happen easily:
1. Web up an enemy, and then do a 3 hit combo. 9 times out of 10, on the third hit, the enemy will be marked ready for a signature move.
2. Upgrade your web strike ability (RB) and unlock the option that allows you to use objects. This will allow you to use various items in the environment (like dumpsters) and hurl them at groups of enemies. You can pick out the items you can use as projectiles while in Web Rush mode - these items show up as a bluish color. Once an object is properly tossed into a crowd, the enemies will be stunned and ready for the signature move.
3. Fully upgrade your web strike ability (RB). This will unlock the "Stun" option, which is really a 4 hit combo at the end of your web strike (RB attack + X). After the assault, the enemy will be on the ground and marked ready for the signature move.
4. Fully upgrade your web pull ability (LB). This will unlock the "Clothsline" option (LB + X). After this, the enemy will be down again and ready for the signature move.
5. Fully Upgrade your Seismic Blast ability (Hold B). The last option for this ability lets you knock normal enemies down, and stunned, ready for a signature move. A good place to really farm this, as well as farming experience, is during the final chapter (Maximum Carnage).
*** Spoiler - click to reveal ***
After you save the initial 3 civilians at the beginning of the level, there will be a cutscene displaying a scared Menken being attacked by several Carnage-infected patients.
It is advised to take out the enemies elevated around Menken BEFORE you go for the infected ones. There are a few soldiers and gliders up there, and they can be annoying (via shooting you) if you let them remain conscious.
Anyway, take them out, then seismic blast the infected crowd. You can get ~5 signature moves done before they get up. Then rinse and repeat. More enemies will spawn as you take these guys out, and they will tend to run towards Menken if you are out of range.
If you want to farm exp for your suits, after you take out the last infected enemy, wait for the exp to show up on your progress bar, then simply hit start, and restart the mission at the checkpoint. You can average anywhere between 2,500 and 3,500 exp per Menken run, and level a suit from level 1 to 8 (max level) in ~20 mins.
Questions, corrections and neg vote justifications below.