The OP might have been a little more picky about the video they (decided to) rely on.
Cutting those stays with a blade is poor (unsafe) practice.
I offer a few additional points, with which many looking will be all too familiar, offering added value: missing in the video.
1) Clean the attachment areas first. Once fitted the crannies formed are super hard to reach.
2) Clean the threaded braze-in(?) in the chainstay bridge.
3) Use a low profile button head (hex or slotted) not a chunky bolt ((don't) see video). This maximises clearance there.
4) When securing the mudguard bridge onto the seatstay bridge with C
21 brake (calipers) the bridge will have to go in between the calipers an the seatstay (ie behind the) bridge.
5) When securing the stays at the drop out braze-ons, I use a washer in between the drop out and the stay eye, as well as between the the stay eye and the bolt.
6) Care needed to choose a bolt length on the rear right drop out to ensure chain is not fouled.
7) Cut the stays to length safely with a junior hacksaw and a gloved hand.
8) When securing the front slotted tab onto the fork crown there can be two connected issues:
a. The top of the tab interferes with the headset. One can bend it sharply forward: I cut it off: it is secured by friction and two tanges are fine.
View attachment 684346
b. To get maximum clearance you often have to file (extend) the slot closer to the mudguard.
9) For quality 'mud'-stopping good people add a flexible extension to both front and rear guards (fixing methods and aesthetics vary).
10) Disc brakes can cause routing challenges for the left hand stays.
HTH