Garrison's trades starts with Garrison in Ninja - Gunslinger map (182, 85). 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 Garrison in Ninja - Gunslinger map (182, 85)
When Base class is Gunslinger; Base level 54 or lower; Room in your inventory for what you get Choose “Yes, make it for me immediately.” Have with you 50 ×
Steel, 20 ×
Rusty Screw, 1 ×
Oridecon, 50 ×
Coal, 3 ×
Elunium, 30,000 zenyIt takes 1 ×
Oridecon, 3 ×
Elunium, 50 ×
Steel, 50 ×
Coal, 20 ×
Rusty Screw, 30,000 zenyYou get 1 ×
Garrison [1]Well, do you want one?.
Hmm, looks like the correct amount of materials and Zeny.
Here's a Garrison~
If you need one again, come and see me~
Script:
npc/quests/gunslinger_quests.txt:311
2 Talk to Garrison in Ninja - Gunslinger map (182, 85)
When Base class is Gunslinger; Base level 54 or lower; Script check: not checkweight(13105, 1) != 1 Choose “Yes, make it for me immediately.” Have with you 10 ×
Steel, 10 ×
Rusty Screw, 10 ×
Emveretarcon, 30 ×
Coal, 1 ×
Elunium, 1 ×
Garrison [1]It takes 10 ×
Steel, 10 ×
Emveretarcon, 10 ×
Rusty Screw, 1 ×
Elunium, 30 ×
Coal, 1 ×
Garrison [1]You get 1 ×
Garrison [2]Well, do you want one?.
Hmm, looks like the correct amount of materials and Zeny.
Here's a modified Garrison with a slot.
If you need one again, come and see me~
Script:
npc/quests/gunslinger_quests.txt:370
Rewards over the whole quest
1 ×
Garrison [1]; 1 ×
Garrison [2]
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.