Just ride easy to start, progressively raising your level.
If its for a race particularly a crit or a TT, I follow a bit of a plan with some spinning building up to some short intense intervals.
There's no need to stretch before you ride. In fact there's pretty decent data that stretching cold muscles won't decrease injury, and may actually make you slower.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.