A method of controlling intravascular sheaths including the steps of: a) obtaining a guide assembly having a body with a tubular wall bounding a passageway, the body having a length between axially spaced ends and an outer surface that tapers towards each of the spaced ends, the body flexible at each of the spaced ends to allow the spaced ends to bend transversely to the length of the body； b) directing the guide assembly into an operative state within a human body vessel； c) obtaining a first sheath having a tubular body extending around a passageway； and d) relatively moving the guide assembly body and first sheath to thereby direct the guide assembly body into the passageway on the first sheath and thereafter guidingly sliding the first sheath lengthwise against the outer surface of the guide assembly body to: a) advance the first sheath into the human body vessel； or b) withdraw the first sheath.
