Turn off the make-up gas, turn down the air to half its usual flow. You now have a fuel rich mixture to make ignition easier. The best tool for the next step is some kind of gas cigarette lighter or other lighter with spark, not flint, ignition. Hold the tip of the lighter over the top of the FID, close to where the exhaust gasses come out and light it. If all goes well you will hear anything from a quiet pop to a sharp crack as the FID ignites. Check that you have a flame by holding a cold shiny surface at the exhaust and looking for condensation. Once the flame is burning restore the air and make-up gas flows. You might need to play with flows and technique a little bit, but it should work.
If you only have a flint lighter then get the flame of the lighter lit before you put it near the FID - you do not want little bits of lighter flint falling down inside the detector.
Good luck.
Peter