If you mean making the start of a sensorgram beginning at 0.0 (zero response and zero time) there are two function buttons depicting a two headed arrow horizontal and vertical.
Select the region, press the button for zeroing in response and select the upper option.
Select the region, press the button for zeroing in time.
For each line you have the slide the cross marker to the start of the sensorgram. All done select OK
Subtracting lines you have to do with one set at the time. That why I like srubber so much!
Arnoud