OK. Harry I hope you have penciled these borderlines.
If yes...prepare some the Mid Stone paint and fill these spots that have to be of the colour.Remember that if there are edges ( wings, fin, rudder etc...) you should paint from the middle of the areas to their edges.
Start with the starboard wing. You can hold the model with your hand keeping the model "nose". There is one small spot of the Mid Stone only and you can paint this as the last area.If you apply the paint a little bit outside these borderlines.Don't worry about it. You will correct it with the Earth Brown. However try to do it as exactly as you can. Of course you can paint on these masks.