Quality parts quality tools, know how. experience.
If you install an engine part, you read, check, measure, install and retest.
A proper tool for this has a few items needed.
a ground electrode bender works best because todays spark plugs are delicate.
You can get a gapper when you buy plugs, ask the correct spec., and give the correct vehicle info.
The wire type is accurate and they often include the electrode bender. Lisle tools, sears craftsman, there are many, they are cheap, and anyone who can pay attention can figure it out.
LISLE TOOLS 67900 $2.50 Autohaus.
You don't say what you are working on, so recheck your work and verify spark and fuel. compression and timing.
You can update your question with your vehicle info, and engine and system type.
Trouble codes, etc.
Then you can get answers for your no start/ hard start. please.