For this, you have to create one of each of the custom weapons on a workbench.
The weapons are; Shishkebab, Bottlecap Mine, Dart Gun, Nuka-Grenade, Railway Rifle, Rock-it Launcher and Deathclaw Gauntlet.
Here is a list of the ingredients needed and the location to find the plans;
[Note that there is a perk in the DLC called Warmonger, which is available at level 26, which awards the plans, without having to go on an extensive treasure hunt.]
Shishkebab;
[Motorcycle Gas Tank & Handbrake, Lawnmower Blade, Pilot Light]
-Sold by Lucky Harith
-Brotherhood Outcast Shack,
-Blood Ties. Possible reward
Dart Gun;
[Paint Gun, Radscorpion Poison Gland, Toy Car, Surgical Tubing]
-Head of State. Possible reward
-MDPL-05 Power Station, near skeleton
-Tenpenny Tower, Sold by Lydia Montenegro
Bottlecap Mine;
[Lunchbox, Cherry Bomb, Sensor Module, 10 Bottle Caps]
-Tenpenny Tower, Inside Dashwood's Safe
-The Wasteland Survival Guide, awarded by Moira Brown.
-Little Lamplight, Sold by Knick Knock
-Jocko's Pop & Gas Stop
Rock-It Launcher;
[Vacuum Cleaner, Leaf Blower, Firehose Nozzle, Conductor]
-Trouble on the Homefront. Behind your
mother's favorite Bible verse plaque [Average] in your fathers lab.
- Craterside Supplies, sold by Moira Brown
-Sold by Crazy Wolfgang
--In the bridge tower third-floor armory [Very Hard]
Nuka Grenade;
[Nuka-Cola Quantum, Tin can, Turpentine, Abraxo Cleaner]
-Hidden inside the Yoo Guai Cave, at cliffside caverns
-Sold by Doc Hoff
-The Nuka-Cola Challenge. Possible reward
Deathclaw Gauntlet;
[Wonder Glue, Leather Belt, Medical Brace, Deathclaw Hand]
-Minor Quest, Council Seat. Possible reward
-On body of Wastelander beside deathclaw
-F. Scott Key Trail & Campground
Railway Rifle;
[Fission Battery, Pressure Cooker, Steam Gauge Assembly, Crutch]
-Stealing Independance. Possible reward
-Underworld Outfitters, sold by Tulip
-MDPL-13 Power Station
If you are having trouble locating the materiaals, you could use
http://fallout.wikia.com/wiki/Fallout_3_component_locations as an aid.