I have done this two ways. The first is very OCD. The second is faster. Both seem to work well.
1. I obtained heavy duty, self adhesive, velcro hook and loop tape on Amazon - in black. It is 2 inches wide and comes in a short roll. Finding an appropriately sized jar top, I used a razor knife to cut perfectly circular SOFT patches, to be mounted on my rucksack.
Next, I found a tailor with a free-arm sewing machine who could sew the sift side of the velcro patch to the rucksack, in the locations I pinned them. When I got it home, I seam-seamed the many very tiny holes made during the sewing process. This is similar to how tactical packs are manufactured with soft panels to affix patches on them.
Third, I used the existing adhesive backing of the HOOK side of the velcro tape to affix the patch. I then used a sharp scissors to trim the squared corners from the hook side of the velcro. I left each patch under a very heavy pile of books, overnight, to ensure a tight bond.
The patches can be attached and removed to / from the rucksack as desired. They are not easy to remove. Someone who wanted to take the patch would have to work at it - then there is that characteristic RIIIIP noise that velcro makes.
2. The other, expedient manner to do this is to simply determine where each patch will go. Then place the patches, upside down, on a scrap piece of cardboard. Use the 3M heavy duty spray adhesive to coat the reverse of each patch until they are property coated. Follow the directions - but only spray the adhesive on the patch, not the rucksack - regardless of what the instructions might say.
Once the desired tackiness is achieved, place the patches carefully on your pack. BE CAREFUL. With this adhesive, you get one go.
The advantage to method #2 is the speed with which you can attach patches, and the difficulty removing them. This is important when you stop at a cafe and leave your rucksack outside.
The advantage to the first method is that you can easily remove and preserve the patches if you desire to remove them for any reason. I did so when I donated my gently used rucksacks at the Pilgrim House. Someone else can choose to make use of the sewn-on soft patch spots.
Hope this helps,