What you do and how depends on what tools you have, and how much
sheet there is.
If you bought it new from a bullion dealer, he will surely take it
back if you havnt damaged it and exchange for the 1.6 mm you want,
with a handling charge of 10%.
As a newbie your skill level does limit you, however, why not
consider it a learning exercise? for example if it were me, id cut a
strip say 10mm wide, scribe 2 lines 2.5mm in from each edge, fold
inwards on these lines so you have a 1.6mm thick strip with a seam
down the middle., on the inside.
Then take it from there to make up a ring, as you know to make a
ring of say 25mm internal dia you need a strip say 80 mm long.
Basic sheet metal work.
You are the smith that needs the learning exercise.Come back and
describe how you folded the sheet accurately, and what tooling you
made up to do this, assuming you planning on making up 20 rings in a