You didn’t say what program you’re using. If you have Photoshop:
Press D to set your colors to black and white.
Create new layer over your watch face. Name it Sphere. Fill with black.
Move the flare to where you want it. Probably upper left. Not all the way on either edge. Set brightness to 115%. Use the first default lens and click OK.
Filter->Distort->Polar Coordinates. Select “Polar to Rectangular”.
Image->Image Rotation->Flip canvas vertically.
Filter->Distort->Polar Coordinates. This time, select “Rectangular to Polar” and click OK.
Select the sphere you just created with the elliptical marquee tool. Select->Inverse. Click delete to remove the background and isolate the sphere. Press Command+D to deselect.
change the blend mode of Sphere 1 from Normal to Screen. There’s your glass dome.
Select your watchface layer. Make a backup copy just in case. You might need to flip it vertically to get it right.
Filter->Distort->Spherize. This part is arbitrary; start with 25% and adjust to your liking.
That’s it. If you’re using something other than Photoshop, find someone else because I’m not typing all that out again