I've tried understanding how to use methods, but still am unclear on how to read their "spec" in get-member.
Is there a guide on how to understand how the descriptions for methods should be read?
string ToString(), string ToString(string format), string ToString(System.IFormatProvider provider), string
doesn't tell me how I should can/use this method.
Get-Member can't really give you documentation. Search MSDN for the type name, e.g., System.String. That'll get you the class documentation. Many methods, like ToString, provide multiple means of using them, called overrides. Think of them as different parameter sets for the method. The docs will include them all.