>>2614795>without damaging the bag itselfDepends on what you mean. I sew velcro backings onto the bag that I slap the patches onto. That will ruin waterproofing, but if you don't have that then sewing really isn't going to hurt your bag. But I also only put patches on that I'm not going to be removing until I retire the bag, limiting the amount of stress I put on the velcro and sewjob to only a handful of times of removing the patch for whatever reason. There are adhesive backed velcro backings that, if you plan to have a permanent patch until you retire your bag, I'd suggest sticking and then sewing on, but thats just me being paranoid that it might get ripped off while I'm hiking. I don't plan on removing the adhesive or threads ever, ymmv. I picked up cheap (<10$) velcro backings from walmart that I cut to size. Also picked up my first needle and thread there a week ago, so if you've never sewn and are worried that you'll damage something, you won't, don't worry.
tldr
backpack -> velcro backing (sew/adhesive/both) -> patch
