When you don't have flag claimed, they show up as white/grey-ish octagons on ground. When you shoot the octagon, flag pops up in middle and you have claimed it. You get achievement for finding 10 of them, but there are more than 10. On level select-screen, you can see flag at the bottom where the level is, when you have claimed it.

12 - Prison Escape. Pick Teleporter and go where portal is. Use Teleport to get on top of the tall platform. Flag is on there, right above portal.

14 - Blink. Pick Teleporter and go on second or third platform. Use the edges that come out of the pillar to get on top of the pillar. Flag is on top of the second pillar.

16 - Peekaboo. Pick Teleporter and teleport through hole in spike-room. Then teleport to the top of tower. Flag is right there on the side where you are before teleporting.

20 - Faster Than Death. Go all the way to the end and pick Teleport. Then teleport to the top of the map, either on walls or the metal fencing. Flag is right above spawn on a chimney.

25 - Too Far. Pick Anti-Gravity and go on top of the pillar in front of you. Flag is right there at top.

39 - In Between. Pick Teleport and go back to where you spawn, then teleport on top of the level in front of you. Teleport your way all the way to the right of the level at top.

43 - High Power. Use jump pad to get on top, then get Teleporter. Use Teleporter to get on top of the cage. Flag is on the left corner, nearest to spawn.

44 - Going Under. Pick Anti-Gravity at start and hover over the building in front of you. Flag is on top of the chimney.

50 - Heist. Pick Teleporter and teleport your way to the top using edges in front of tower on the side with longest platform/fire trigger.

55 - Flytrap. Pick Teleport and use it to get above where you spawn. Flag will be on the end of the platform.

57 - Firewall. Pick Spawn - Platform and make your way to the top of the building you spawn in, whether you go to the end or just go under the building at start. Flag is at the end of the building, above where you spawn.

61 - Sticky Walls. Use Teleport to get to the top of the left wall. Flag is on the end nearest to spawn

63 - Wall Jump. Use Teleport to get to the portal. Don't enter the portal and teleport to wall next to it. Flag is on the end nearest to spawn

66 - Spider. Use Teleport to teleport on top of the wall in front of you. Flag is on the end nearest to spawn.

79 - Sleigh. When you spawn in, just jump few meters back, so you are on top of the platform. Flag is right there, tilted in the platform.

80 - Boardsword. Make your way all the way to the end. When you are near the portal, use Teleport to teleport to platform above you, then use it to teleport on top of sword. Flag will be all the way at top.

-H1 - Guitar. Make your way to the portal at the end of the level. Flag is on right side of portal platform.

-H5 - Farming. Make your way to the portal at the top of the map. Once there, flag is on the left side, looking from where you land.

-H6 - Harrier. Pick Teleport and teleport your way to left part of platform in front of you. Flag will be on the top.

-H8 - Grappling Hook. Pick Teleport and teleport your way to the end of the level. On the portal platform, use Teleport to teleport on top of the platform behind you, the one you used to get on portal platform. Flag is at the top.

Credits:
snp_: For finding numbers 16, 25, 39, 43, 50, 79
Judgy: For finding numbers H1, H8