To unlock Ghost Mode you'll need to complete all challenges on any 1 of the mountain runs, Tannenstein, Eisige Allee trail is easiest one.
Unlock the Black Slope by completing all the challenges on the Blue Slope. Then, complete all the black slope challenges too. This will unlock the "Show Ghost" feature and the "Boooh!" achievement will pop.
Once unlocked, you can race against your best times in Ghost Mode for another achievement.
This video has all 45 achievements, BOOOH!! Achievement starts at 38:13
Achievements List
0:00 – Intro & Tutorial Trail
0:42 – Unstoppable Achievement & Training Day Achievement
2:37 – Start of Just Trailing Achievement (Finish all Tannenstein Blue Slope Challenges)
3:28 – Tannenstein Slope 1 – Blue Challenges & It’s all Downhill from here Achievement
5:36 - Tannenstein Slope 2 – Blue Challenges
7:53 - Tannenstein Slope 3 – Blue Challenges & Ramming Through Achievement
10:41 – Tannenstein Slope 4 – Blue Challenges & The Monte is Calling Achievement
13:53 – Start of Off the Beaten Trails Achievement (Finish all Monte Guanaco Blue Slope Challenges)
14:13 – Monte Guanaco Slope 1 – Blue Challenges
17:02 – Monte Guanaco Slope 2 – Blue Challenges
19:37 – Monte Guanaco Slope 3 – Blue Challenges
21:47 – Monte Guanaco Slope 4 – Blue Challenges & Riding High Achievement & Snake Charmer Achievement
25:24 – Start of Fast and Freezing Achievement (Finish all Sierra Gelida Blue Slope Challenges)
25:44 – Sierra Gelida Slope 1 – Blue Challenges
28:04 – Sierra Gelida Slope 2 – Blue Challenges
31:53 – Sierra Gelida Slope 3 – Blue Challenges
34:42 – Sierra Gelida Slope 4 – Blue Challenges (Fast and Freezing Achievement end)
37:53 – Start of Born to Ride Achievement (Finish all Tannenstein Black Slope Challenges)
38:13 – Tannenstein Slope 1 – Black Challenges & Boooh! Achievement & RUN, Cheetah Run!! Achievement
40:31 – Tannenstein Slope 2 – Black Challenges
43:11 – Tannenstein Slope 3 – Black Challenges
45:52 – Tannenstein Slope 4 – Black Challenges (Born to Ride Achievement End)
49:10 – Start of Offroad King Achievement (Finish all Monte Guanaco Black Slope Challenges)
49:30 – Monte Guanaco Slope 1 – Black Challenges
52:02 – Monte Guanaco Slope 2 – Black Challenges & Free Like a Bird Achievement
54:40 – Monte Guanaco Slope 3 – Black Challenges
57:07 – Monte Guanaco Slope 4 – Black Challenges (End of Offroad King Achievement)
1:00:16 – Start of Master of the Snowy Mountains Achievement (Finish all Sierra Gelida Black Slope Challenges)
1:00:36 – Sierra Gelida Slope 1 – Black Challenges
1:03:10 – Sierra Gelida Slope 2 – Black Challenges
1:06:38 – Sierra Gelida Slope 3 – Black Challenges
1:11:27 – Sierra Gelida Slope 4 – Black Challenges (End of Master of the Snowy Mountains Achievement)
1:15:12 – Perfect Ride Achievement
1:17:28 – No Brakes, High Stakes Achievement
1:19:46 – Sdrawkcab Achievement
1:20:55 – Smash ‘n’ Grab Achievement
1:21:35 – Holding On Achievement & Jump to Safety Achievement
1:22:17 – Have a Save Flip Achievement & Flip Like a Switch Achievement
1:23:12 – Spin the Rider Achievement
1:23:56 – You Spin My Head Right Round Achievement
1:24:18 – Flippin’ Awesome Achievement
1:24:47 – Mix it Up Achievement
1:25:16 – Master Trickster Achievement
1:26:18 – Free Fallin Achievement
1:26:57 – Like Having Wings Achievement & Master of the Element Achievement
1:28:24 – Ghost Buster Achievement
1:31:13 – Under the Pines Achievement & Peace and Quiet Achievement
1:34:48 – Path to Paradise Achievement
1:37:46 – Lonely Places Achievement
1:41:15 – The Journey is the Destination Achievement
1:41:43 – To the Races Achievement
1:42:15 – Watch me Ride Achievement
1:42:45 – Race like you mean it Achievement
1:43:19 – Badge of Honor Achievement
1:43:58 – With a Little Help from my Friends Achievement
1:44:54 – Teamwork Makes the Dream Work Achievement
1:47:22 – We are the Champions Achievement