I have doe this before.
1st you have to pre cut gear so size leaving a inch for the long joint overlap
plus half inch each end to trim – if you go a bit squiffy
2. you have to apply application tape to the mirror gear, as one it will stiffen it up and two, if you don’t you will scratch the grandmother out of the job.
3 after you have covered your pre cut panels, mark the centres at the end, don’t however strike a line right the way through or you will see it in the finished job
4. lay the tube flat on the floor, put a bubble on it to find the tops at ech end – mark the tops each end on the end.
5. flip the mirror over on the bench and tear out a 6 inch strip through the centre
6. all you need is a light dust of soapy along the top.
proceed to apply one half, then the other.
p.s. if you try and do it in one hit from one side, all the way around and you are a few degrees off, – by the time you get all the way around you might run off
doing each half, halfs the error, plus you minimise the risk of creasing the gear