It's not a job I would do without a torque wrench.
For a brand new spark plug (never been fitted before so the washer is not compressed), in an alloy headed engine like ours (not a cast iron head), the 14mm thread diameter plugs require:
18-22lb/ft torque to do them up (25-35Nm). Source: Spark-Plugs.co.uk (
here)
IF you are determined to fit them yourself, and you don't have a torque wrench, then on the box of NGK plugs in my drawer, there is a diagram which shows for newly unfitted plugs that you do the plug up finger tight, then:
For brand new unfitted plugs: Turn 1/16 of a revolution using deep plug socket / wrench (minute hand from 00mins on a clock face to about 5 minutes past the hour)
For refitted plugs (post checking): Turn 1/2 to 2/3's of a revolution using deep plug socket / wrench (minute hand from 00 mins on a clock face to between 30minutes - 40 minutes past the hour)
Whatever method ensure consistency of approach for each plug, and spray a little electrical contact cleaner onto all contact points of plug / coil pack for best quality connection.
HOWEVER please be mindful alloy heads are made for lightness, and therefore lack the material strength composition of cast alloy. Overtightening may not only cause fatal damage to your engine head, but could cause excessive heat transfer from the block into the plug causing the engine to run erratically, and the plug to be exposed to excessive temperatures during operation (tip failure/overheating of ceramic collar). I know this sounds a bit excessive, and is unlikely, but it's better to know the risks in advance.