If you search for images of mirrors you can clip on glasses or helmets, you will see that most come with a clip, a stem and a mirror. Or see the pictures and read the text behind this link (I have no connection to the site, was only a happy reader when I found my now favorite mirror on it.)
I do not know your budget, but I found ordering mirrors online (on one of the big international selling sites) not expensive. Have a look, it might be worth it for starters while you work out what you want to make.
Unless working with an old pair of glasses I would not add anything direct to them. When using a clip on mirror I notice I take it off about half the time I am inside a building and sometimes I need the glasses still when I do not need the mirror.
And consider safety when the mirror breaks, as you can read in the linked text, several producers of mirrors have made 'break away' parts, so that when you fall while using the mirror, it will get away from your eyes and not into them. I feel that the clip on most of the mirrors will help in that.
The stem, while not essential, is important to place the mirror where you get most use out of it. But the stem should be rigid enough for the weight of the mirror and its length, so the mirror will not wobble around when you move your head (or you ride over a bumpy surface.) The stem should also allow you to set your mirror or the connection between the stem and mirror should allow for that.
And as someone who has used several mirrors on several different bikes and with several hats, caps and hoods as well as with hair shorter and longer, I know you will want to adjust the mirror even when you use the same glasses on the same transport, when your head gear or hair changes.
The mirror itself can be glas or a kind of plastic, it should not be too shiny and it should be either flat or a regular shape. It can be convex or concave, the result will be different, either giving you a wider field in a smaller image or a smaller field in a wider image, which is better will depend on where you use it and how good you can take in information.
Shapes that are not regular will distort the image and will be bad, not giving you reliable information.
And again consider the risk of a break of the mirror, you will want a mirror that will not be a risk for your eyes or skin.
I use one clip on mirror at a time, as it is easy enough see see behind me while cycling. (I used to have two mirrors on my recumbent bike but noticed one is enough for almost all situations, especially if it is on the side traffic will pass you.)
Clip on mirrors stand out rather much, I get more questions about my mirror than about my recumbent bike, and I doubt your home made one(s) will be less visible if added outside your glasses.
If you want an option that is not that noticeable, the site in the link above also shows a tiny mirror which is put on the inside of your glasses. I have never seen them in real life, I have heard them described as 'to watch the girls behind you' mirrors. If you want your mirrors all the time to see behind you while not on a bike, you may use those as inspiration.
If home made, you may be able to attach something to the lower edge or corner of your glasses. But it will be very hard to get them orientated right. It might be worth it not to make your own and order this kind of commercial mirrors, as those are adjustable.