I'm following the book Tools - by Chris Platt where he suggests that it is really important to use a cork-backed steel ruler.
I have an 18" cork-backed steel ruler, and also a 6" one. The cork is maybe 1/16" thick, and it does help prevent the ruler from sliding around on slippery surfaces. Wood, however, is not a particularly slippery surface, and steel rulers don't tend to slide around on it. In fact, if I'm using one of those cork-backed rulers as a straight edge, I tend to turn it over so that the steel face is against the wood, and slipping is never a problem.
The book you're looking at isn't a woodworking book -- it's broader than that. Charles Platt is a pretty reliable source, but I think you have to take his suggestion that a cork backed ruler is "more important than it sounds" in the context of someone who works with a wider variety of materials, not just wood.
If you're mainly doing woodworking, I wouldn't worry for a millisecond about the lack of cork backing on your ruler. And no matter what materials you're using, don't get too caught up in having exactly what someone else says is the right collection of tools. If you don't have a cork-backed ruler and your ruler keeps slipping on a surface like sheet metal, just stick a piece of duct tape, gaffer's tape, friction tape, etc. on the back. Call it a "life hack" if you like. ;-)
Being a "maker" is all about finding ways to create what you want using what you have. Consider Platt's advice and the reasons behind it, but don't get caught up in assembling his perfect list of tools, because you are not him.
All that said, you asked about how to make a cork-backed ruler. If you have some very thin cork sheet and a steel ruler, then all you need to do is to cut a piece that's about 1/4" shorter and 1/4" narrower than the ruler you're using, and stick the cork to the ruler. You can use a spray adhesive like 3M 77 or 3M 90, contact cement, or even thin carpet tape or other double-sided tape. I'd stay away from PVA (wood) glue or epoxy, as those are both likely to soak into the cork and harden it, which would defeat the purpose of using cork in the first place. Try to make sure that the whole surface of the cork adheres to the ruler (so that particles don't later get between the cork and ruler).
In the US, you can find thin cork sold as non-adhesive and self-adhesive contact paper, shelf liner, and tape. Check craft stores and home improvement stores. Also, you might want to look more carefully for cork-backed rulers; I think you said that you're in India, and a search for "cork ruler" on amazon.in turns up a number of options.
This is a separate question - but - there is a recommendation that says "The countersink must have only one flute and must be ½" diameter." ... what's up with that - the most commonly available ones on amazon are 5 flute ones
We had a recent question (Why is my countersink bit making hexagonal holes?) about countersinks; the reason Platt wants you to get one with a single flute is to help a beginner avoid problems like the hexagonal holes that result from chatter when used with a handheld drill. He's similarly prescriptive in Make: Electronics where he tells you that your multimeter "must" be digital. He's not expressing a strong opinion; it's just that the book is written with the expectation of a digital meter, and analog meters don't afford the needed resolution. Platt's books are easy to follow, and part of the reason for that is that he sets you up to avoid some of the problems that beginners often run into.