Looking at the pictures below, when you unwrap your sirloin tip it should resemble either the left or right roast, depending if it has the 'cap' on it or not.
The third picture is of a front slice off the roast. If you cut the roast lengthwise as shown, dividing it into two parts left and right, you will have a 'silverside' tip roast and a 'petite sirloin' tip roast. You can put these through netting or tie them up if you wish, or just roast them whole the way they are. If you cut it the 'other way' (front full slice and back full slice) in half that way the rear of the roast is no where as good; this way you get the best on each roast narrowing to the least best. Once you unwrap it if you still have questions, take pics and hold it in the fridge until I get on and can answer them! Thank you for asking however, we're always glad to help! Or, send me an email at email@example.com and I'll get on right then if at all possible.