1. An even number of scrap metal is needed. The RobCo Facility and the Meresti Station are two places with a lot of scrap metal.
2. (Skip if friendly with Outcasts) Collect various items for Casdin, enough to achieve allied status with the Outcasts. The best place for this is the UFO crash site to the north, not only since the loot yield far more points than required to become friendly with the Outcasts, but also because one can simply stop by Meresti Station on the way and loot the scrap metal there.
3. (Skip if friendly with Outcasts) Speak with Defender Morgan to start The Outcast Collection Agent quest.
4. (Skip if friendly with Outcasts) Give Casdin enough items to become friendly with the Outcasts, around 50 points worth of items.
5. Give Casdin all scrap metal in your inventory. This batch is the X Value. Note! This value MUST be an even number (10, 12, 14 etc).
6. Steal back the X Value scrap metal you gave Casdin.
7. Give back all scrap metal except 2 for the best possible profit. This batch is the Y Value.
8. Steal back the Y Value scrap metal from Casdin.
9. Now, attempt to steal one of Casdins basic items, such as his Laser Rifle (or if you've given them to him, the Alien Power Cells). You should get caught (if not, try again). Casdin will then proceed to take back his scrap metal, but will steal back both the X Value and the Y Value. For example, if you gave him an 16 scrap metal, steal them back and then gave him 14 scrap metal, it will say that he removes 14 scrap metal and then 16 scrap metal.
[more cheats for Fallout 3 here]