Keep chinbar closed. Open visor to fullest extent. With the lid facing away from you, grasp the visor with both hands at the lowest point on either side. With the heel of your palms on the rear edge, push forward with the heel of the palm and rotate backwards and upwards.
When they're new and a bit stiff or if you don't whip it out often (ooer missus!), you might find doing one side at a time the easiest using the free hand to steady the lid.
Don't be afraid to use a bit of force.
First time I did it my heart was in my mouth. Now I can swap visors or convert the lid to open face with peak in a few seconds.
To remove the chinbar, similar action except that with the visor off, the chinbar can rotate upwards to a second detent position, then push forwards on each side. Worth doing every so often as a little Vaseline on the hinges keeps the action smooth.