Preface: When it comes to fast creep spread, zerg players should generally spawn new creep tumours at the very edge of the creep. This needs a lot of mouse accuracy to do quickly, and is more difficult when spreading multiple creep tumours quickly.
1. Go to Menu -> Options -> Hotkeys -> Global -> Unit Management -> Choose Ability or A.I. Target. Set the Alternate hotkey to C (or if you changed your creep tumour's spawn new creep tumour ability, set it to the same hotkey as that)
2. Go to your OS control panel equivalent and look for keyboard settings. Reduce key repeat delay to the minimum setting and increase key repeat speed to the maximum setting.
Double-click your creep tumours, then hold C while you wave your mouse where you want the creep tumours to go.
The waving of the mouse has several techniques, depending on the position of your current creep tumours and which direction(s) you want to expand in.
1. One creep tumour: wave from outside the creep to inside the creep boundary in the direction you want to spread. Result is furthest creep tumour in correct direction.
2. Multiple creep tumours and want to spread out: wave from outside the creep in a zigzag inwards to the creep. Result is furthest creep tumours in spread out directions.
3. Multiple creep tumours with some further back and some further forward, but you want them all to go in a certain direction: same as one tumour, wave from outside the creep to inside the creep boundary until all tumours are placed.
4. Multiple creep tumours and can't advance further: wave inside the creep to thicken the tumours.
replay in real game: (gold ZvT: i'm terrible but the easier creep spread helps!)
Khaldor made a video tutorial that mentions this:
// credit to ManiacTheZealot for suggestion of using the same key for spawn creep tumour and Choose Ability or A.I. Target, also for noticing that there is no need to hold shift when placing tumours.
// thanks to Existor for banner!
Bonus: Set your infestor spawn infested terran ability Alternate hotkey to C as well for easy infested terran spam.