I don't know how to ensure that your head is tilted at the same angle, other than telling you to take a lot of photos covering multiple degrees of neck flexion and extension. Then choose the result that is most head-on. But here are photography tips that will help.

Use a tripod and get a real camera with a long lens. If you are just using a cellphone camera, it has a short and wide lens, which means it has to be very close to you. The closer the lens is, the more perspective there is. Perspective will magnify asymmetries, and also magnify misalignment of the camera. Look at this picture for an example of how different focal lengths of lenses cause perspective distortion and magnify asymmetries. The closer the camera is, the hard it is to see the whole face.

You could even take the exact same camera you have now, and move it further away. This will reduce perspective effects. However, you will have to crop more of the image, resulting in lower resolution (which is fine as long as your camera has plenty of resolution).

The other thing that can do in post is draw lines between landmarks on profile photos, and then rotate photos to equalize them. This will allow you to line up your face to compare, even if neck posture is different (which hopefully it will be, because we want to see neck posture improve).

Posted by: Abdulrahman

Keep lighting consistent; try to take all the pictures in the same room using the same light source. For example, if you use a window light always take the pictures the same hour (like noon). This will keep the direction and color of the light consistent. 

Keep distance and height consistent; try to take the picture 1.5 to 2 meters away from the camera. This will eliminate perspective distortion. Take them at the same height, doing otherwise can distort the relative size of your facial features and make comparing them hard. 

Keep camera level with the ground if you can; this is a hard one but really helps when trying to judge forward head posture which is really the first step in analyzing any picture. Some phone cameras have an internal level feature, this can be really handy. 

If using your camera phone, it's ok if there is allot of empty space around you. Camera phones have very wide angle lenses. Just make sure the resolution of the phone is set highest. After you have taken the picture it can be cropped.

I can't find the second post I made that goes into more details.