It's definitely better to use the u shape tools but with the nails, it can be done also, push the nails into each hole, then you have to hold both with each hand, so both left ones with your left hand, both right ones with your right hand, then push inwards (pretty sure, can't remember honestly, haven't done this for probably 5 years now) and while pushing inward (like in towards the deck, not to the back of the deck), pull the unit forward while holding the pressure.
Personally, for the cost of the tool, go buy one...... I was in a pinch and did it with nails at the time (late night, no Walmart even remotely close to where I was living), and just threw in my 2 cents on this thread as an 'alternative' to the tool.....