You need a burn fuel, for winter we used a mix of 3 parts diesel 2 parts Gasoline in a drip torch. If you don't have a drip torch you can use a common sprayer and spray the under side of the pile good then light it. A few old tires in the mix will help, fill the tire with the burn fuel it will help maintain the heat till the pile gets going.
Placing the pile in a hole sometimes cuts off the air flow, best to start the fire on one end of the pile so oxygen can be drawn in, the fire will then proceed to consume any combustible fuel. Once the pile gets going it will progress to the other parts of the pile that you already sprayed.
The way to start a fire when starting it with a drip torch or sprayer is to start a small fire and light the torch or with a sprayer is spray a trail to the main burn start a small fire outside the main burn, that will start the main burn and will keep you a safe distance away, The burn fuel mix doesn't have the fuel flash point that you would have with using with gas.
Best to burn a pile when the cut down material has been cut down about 15 or 20 days because the sap in the wood has fermented which creates alcohol and will help accelerate the burn.