There are 27 fairly easy milestones. With the help of this guide you should get everything in at least 12 hours. After completing this guide, you should have unlocked 420 gamepoints. Good luck!
1. Complete all challenges
Simple for those who played skate. for a long time: Complete & win every race, contest, photo session, pro challenge, follow me and own all spots. After you completed everything, this will unlock along with several other milestones: "Owned a spot", "Owned all spots", "Complete every pro challenge", "Won all photo challenges", "Won all film challenges", "First in all contests" & "First in all races". After finishing everything, you'll gain the "Keep Playing" achievement.
Note: I'm sure you won't unlock this as the first milestone, but when you completed your first milestone; you earn your first achievement. The achievement will come during the career pretty soon, so you shouldn't be bothered about this one.
2. Cause a chase
See "Cause 50 Chases (PEST)"
3. Cause 5 chases
See "Cause 50 chases (PEST)"
4. Cause 25 chases
See "Cause 50 chases (PEST)"
5. Cause 50 chases (PEST)
To cause a chase, skate in a "No-Skate"-zone until the security guy starts following you. You'll notice he follows you when a red arrow is visible on your screen. For these milestones, you don't have to escape but I do recommend it for other milestones as "Escape from a chase", "Escape from 5 chases", "Escape from 25 chases" & "Escape from 50 chases (HOUDINI)".
6. Escape from a chase
See "Escape from 50 chases (HOUDINI)"
7. Escape from 5 chases
See "Escape from 50 chases (HOUDINI)"
8. Escape from 25 chases
See "Escape from 50 chases (HOUDINI)"
9. Escape from 50 chases (HOUDINI)
To cause a chase, skate in a "No-Skate"-zone until the security guy starts following you. You'll notice he follows you when a red arrow is visible on your screen. Escape to make it count. A really good place to trigger & escape is going to the Library, the spot where the Pro-challenge is from Jerry Hsu, where you get the chance to get sponsored by Enjoi. There are 2 small red zones (one on your left & one on your right), which can easily trigger the guards to chase you. Just skate from left to right unlock you unlock this milestone.
10. Spent 1 hour in No-Skate Zones
See "Spent 3 hours in No-Skate Zones"
11. Spent 3 hours in No-Skate Zones
An easy way to get this is go to the back of the big building in the plaza (where the security dude doesn't walk) and stay they put for at least 3 hours. You can leave your console running while you go eating, sleeping, ... and this will still count because it says 'spent' and not 'skated'. Make sure your controller doesn't fall away, because that will pause the game.
12. Played skate. for 12 hours
If you didn't get it while completing career, you can use the same method as described in "Spent 3 hours in No-Skate Zones". Just make sure your controller doesn't fall away. If you use this method, you'll get the "Afternoon Freeskater" achievement.
13. Break 100 bones
See "Break each bone 10 times"
14. Break each bone 10 times
An easy way to do this is going to the X-Games park & take the elevator to big ramp. Set a session marker, go down, bail out & repeat. Another easy way to get this is going downhill and skating into cars. After you get milestone #13 & #14, the achievement "Uninsurable" will unlock.
Note: Beating 14 milestones means that you are halfway the milestones. As a reward, you'll get another achievement. This should come during regular gameplay.
15. A minute of air time
This shouldn't be to hard since everything counts when you are of the ground. One flip trick mostly counts as 0.5 seconds. Do 120 of them to get this milestone. When you are of a ramp, the normal air time is about 2-3 seconds (although you can get a lot more by building speed). This will come during the story, so don't worry
16. Owned a spot
See "Owned all the spots"
17. Owned all the spots
There are a total of 20 spots in this game. When you discover a spot, you'll see on the right "Beat ... points". Just beat the score & the spot is owned. After completing all spots, the "Urban Legend" achievement will unlock. See the video below for the correct locations. Credit goes out to Jonathanl5660 for uploading this video.
18. Do every flip trick
See "Do every trick"
19. Do every trick
Do every trick possible to earn this. You have 3 sort of tricks: flips, grabs & grinds. These tricks are divided in basic, medium & advanced. Please note that all tricks can be done goofy & regular. When you decided to let your skater skate regular & you stand switch after performing a 180, flick it in the goofy directions.
NOTE: ollie, manual & nose manual need to be done as well! After doing every trick, the "Tricked Out"-achievement will unlock.
Note: While completing your trickbook, you'll need to do a Rocket Air. If you tweaked it long enough, another achievement will pop up whether you bail or not. The best place to do this, is skating down the big vert in the X-Games area.
20. Complete every pro challange
To do this, you need to beat every challenge that is marked blue on the map. The most difficult one for me was the "Rob & Big"-challenge where you have to do the 360 Flip FS Crooked while Big Black is walking around. You have to beat each pro on a game of S.K.A.T.E as well to get this, but this is very simple using 540 spins. After beating the first pro, you'll get your first achievement (see above since it's story related) & after beating the crap out of the pros, the achievement "Fanboy" will pop up.
21. Complete all photo challenges
To do this you need to beat all challenges for Skateboard Mag. & Trasher. If you get on the both covers, the most things are done. Afterwards clean up & beat all photo challenges left on your map. "Photogenic"-achievement unlocked.
22. Won all film challlenges
To do this you need to beat all film challenges for Skateboard Mag. & Trasher. After you complete the sponsor films, the most things are done. Afterwards clean up until the "Without Footage it's Fiction" achievement pops up.
23. Skated 10,000 meter
See "Skate 100,000 meter"
24. Skate 100,000 meter
This is cumulative. If you really skate towards each challenge, you should get this in no time. 'Deathraces' & 'Follow Me'-assignments count towards this milestone!
25. First in all contest
Just finish each street contest, tranny contest or jam in first place to earn this. After winning everything, the "Contest Pro" unlocks. The X-Games do count in this milestone as well as for the achievement.
26. First in all races
Just finish each race in first place to earn this. After winning each race, the "King of the Highway" achievement unlocks.
27. Grind an accumulated 1,000 meter
As it says, it's cumulative. Just grind long & many to get this fast.
After getting this 27 milestones, you'll gain your final milestone-related achievement.
Edited on 2011.01.29, included the TA-links for corresponding achievements. This solution will be readable in the skate.-guide as well.