1. Buy Unexpanded Tailored Transcendent Backpack in the Bazaar. They're going for 12k on Bristlebane server these days.
2. Do you already have Guide to Transcendent Bags scribed? Then skip to 3.
- Buy aforementioned book from Scribe Zikett in PoK and scribe it. He is in the Find window and right across the open-air walkway from Dogle Pitt (aka Small Bank). If this is your first time (like it was for me), right click the book in your inventory to open it. Click the Scribe button at the bottom of the window displaying the book. It costs 35pp or so.
3. Buy one Inverse Arcane Solution from Tailor Kujen in PoK. He is in the Find window and literally immediately to Dogle's right. It was 104p 9g 9s 9c for me.
4. Combine the unexpanded bag with the arcane solution in a loom. I used the one right in front of Tailor Kujen. It worked for me and I had literally zero Tailoring skill.
5. ???
6. Profit
Source (mostly): https://www.almarsguides.com/eq/leveling/SoD/Locations/PostMercNoobZones/MoreInfo/Tips/UnexpandedBags/