Back Slide skill quest starts with Alcouskou in Thief Guild (154, 128). The steps below are read from the quest's script on this server, in the order the script's quest stage moves through them.
Walkthrough

1 Talk to Alcouskou in Thief Guild (154, 128)
When Base class is Thief; Script check: JobLevel > 34 || BaseJob == Job_Assassin || BaseJob == Job_Rogue Choose “Sand Attack” Have with you 20 ×
Grasshopper's LegIt takes 20 ×
Grasshopper's LegYou get Skill
Back Slide level 1Suk - Suuuk - - -
Great! -At this level,
I am sure you can increase
your skill on your own.
Script:
npc/quests/skills/thief_skills.txt:172
2 Talk to Alcouskou in Thief Guild (154, 128)
When Base class is Thief; Script check: JobLevel > 34 || BaseJob == Job_Assassin || BaseJob == Job_Rogue Choose “Back Slide” Have with you 20 ×
Grasshopper's LegIt takes 20 ×
Grasshopper's LegYou get Skill
Back Slide level 1Suk - Suuuk - - -
Great! -At this level,
I am sure you can increase
your skill on your own.
Script:
npc/quests/skills/thief_skills.txt:172
3 Talk to Alcouskou in Thief Guild (154, 128)
When Base class is Thief; Script check: JobLevel > 19 || BaseJob == Job_Assassin || BaseJob == Job_Rogue Choose “Find Stone” Have with you 1 ×
Zargon, 1 ×
Bear's Footskin, 5 ×
SpawnIt takes 1 ×
Zargon, 1 ×
Bear's Footskin, 5 ×
SpawnYou get Skill
Find Stone level 1You can improve your skill with
practice on your own time.
I hope it aids you in the future.
. . . . . Hope to see you soon
Script:
npc/quests/skills/thief_skills.txt:262
4 Talk to Alcouskou in Thief Guild (154, 128)
When Base class is Thief; Script check: JobLevel > 14 || BaseJob == Job_Assassin || BaseJob == Job_Rogue Choose “Stone Fling” Have with you 2 ×
Garlet, 2 ×
ScellIt takes 2 ×
Garlet, 2 ×
ScellYou get Skill
Stone Fling level 1Okay, that is all there is
to it. How you use it in
the future is up to you ..
I wish you luck!
Script:
npc/quests/skills/thief_skills.txt:345
Rewards over the whole quest
Skill
Back Slide level 1; Skill
Find Stone level 1; Skill
Stone Fling level 1
How this page is made: every step comes from the NPC scripts the server loads for Episode 13.2 (the file and line are under each step). "When" lists the checks the script makes before that branch runs; "Have with you" are the items it counts; experience from getexp is multiplied by the server's 50x quest rate. NPC lines are the script's own text.