To use it, you'll need to have every piece of New Vegas downloadable content installed, including the former pre-order bonus packs. You can download the tiny 88KB mod direct from Sawyer. The mod, spotted by No Mutants Allowed, cuts the level cap, XP gain, health and healing, while also sprucing up a number of weapons and armour.
Intriguingly, these changes come in a mod of Sawyer's making, not any official patch. Sawyer has reworked the balance of Obsidian's post-apocalyptic RPG, amping up the difficulty and tinkering to his own preference.