You collect them from your local newspaper. Some will come in the mail once a month as well since not everyone gets newspapers these days.
I recommend sorting them out in a small filing folder according to type and then keeping them in order of expiration.
I have no experience with this but I did find a helpful article for you that you can use: https://thriftynomads.com/couponing-for-beginners-guide/
I would look over that and plan according to the experts. This is for beginners as well.
I assume you mean physical coupons for stores. If online, just search on Google for specific store coupons.
For brick and mortar stores, sign up to their programs and so on. You'll receive coupons quite often.
Get one of those binders with three rings and the slip sheets that are plastic meant for collecting cards. You just organize them according to date, type, and store. I started doing this earlier this year and it helps a lot. A friend got me into it.
Just go for it. You will want to keep things organized. You can do this using a binder like suggested or you can use a folder. Some people prefer to use their phones to organize coupons via pictures.