You can start from a reference picture. Start by tracing a rough outline of the main shape, work at it until you get the proportions right and the main elements where you want them to be. Use a pale pencil, such as 4H. It is normal for this phase of the drawing to take a lot of time and to be revised multiple times.
Start with the part of the drawing that mostly draw your attention and that will be the main focus point. For me, it was the beak.
Follow with the crest. For this palm cockatoo, the feathers in the crest are numerous and superimposed on each other. Drawing the crest main contours will require patience as you will need to understand the feathers' position and movement from the reference image.
Work the feathers neck.
Work the main bodily lines contours adding in the texture of the feathers. For the wings, study your reference image for the presence of different kinds of feathers. For this palm cockatoo, there were three different kinds of feathers representing three superimposed layers.
Add in more details and shadows. The crest will once again require more detailed work.
Add feathers details and shadows on body.
The wings will have three different feathers textures to reproduce.
Add an extra layer of darker contours to give more precision and depth to your drawing.
Work on finishing shadows touches to model further your drawing and make it more dimensional. Use a paler pencil if you need some final light texture. Know when to stop adding in details to keep your drawing balanced.