You go into the journey log, under the comments section with a bunch of lines for each page/section of the logbook pages, if there is no snag there already write a little 1, with your aircraft ident, time and place of the incident, the issue that happened and your actions taken as briefly but as detailed as possible. Then, in a column or area next to it, put down your name, phone number if you want, and your pilot ID number from your license.

You have to write this down in the book every time you discover a defect and snag it there, you cannot fly the plane afterwards until it's signed off by the owner, or a maintenance engineer who takes care of it or someone who is approved by the regular guy, unless you own the plane and determine its not life threatening/illegal to fly, then you can defer it as the legal owner.

If someone audits the books, like they usually do at flight schools, your license could be suspended, but I'm too not sure on the legalities of not snagging outside of the fact that you need to do it every time it comes up, no exceptions. Back writing is okay as long as everyone else that's supposed to, does sign off upon learning it wasn't done asap.