With Item Picker, center the crosshairs or “plus” on the stroke you’d like to select, then tap the screen with a finger to add that item to your selection. Add more items by dragging the crosshairs to other strokes and tapping the screen to add them. Your selection isn’t finished until you finalize it by using another gesture (such as dragging it about the screen or tapping one of the popup options). If you drag back over a previously selected item, you’ll see a “negative” or “minus” sign appear in place of the crosshairs. Just tap to remove it from your selection.
With Lasso, the same rules apply. Tap+hold+drag to select multiple items at once. Tap+hold+drag other items (begin your selection outside the Selection box) to add them to your selection or to subtract previously selected items from your selection. At any time, you can toggle between Item Picker and Lasso to switch selection methods.
If you aren’t using the Selection tool to select but are using the tap+hold shortcut, remember to keep that tap+hold going the whole time you’re selecting, since a single tap on screen will exit the Selection mode.