Just in advance - since I am making no money from making this DIY for you - please just be fair and tag me if you try it out and do not steal the idea to make profit from it - love you.
What you need:
relatively thin wood ( has to be a little bigger than your mirror,
you can use 5mm mdf as well)
really strong glue (for ex epoxy based)
first lay the mirror down onto your piece of wood (mine is leftover from another mirror, that's why its already half way wavy)
mark where your mirror sits really visible to have a guidance line for your outer and inner frame
then find your outer and inner shape
make sure the inner line is a good bit away from the outer edge the mirror so there is enough wood touching the mirror (that's where its scribbled)
later on you will put glue on these parts
take your saw and cut out the outer shape
for the inner shape: drill a few holes into the wood, a bit away from where the line is.
make enough holes so you can fit the blade of your saw in.
then cut the inner part out - you basically should have a frame now
sand everything smooth
prime your good side (not the side you marked where the mirror goes) with primer paint
once that's dry, paint it your favourite colour and let dry over night
now you basically just glue the frame onto the mirror
put glue on where the mirror would sit in your markings
I used strong wood glue, you can use other - just make sure its really strong and suitable for glass and wood
put you mirror on, the good side facing away from you
make sure its super flat and put some weights on to make it extra secure - you can basically use whatever you like
let dry at least over night - better 48 hours
I did glue on a piece of wood at the back of the mirror as well so I can hang it