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).
Questions, corrections and neg vote justifications below.