Instructions
Create 40 Mithril Casings, 20 Unstable Triggers, and 40 Solid Blasting Powders. The ability to create all of these items can be learned from any Engineering Trainer.
Create 20 Hi-Explosive Bombs using the materials you previously created. This should level you past 255, but this can vary based on your success in receiving points each time you create a bomb.
Create 20 Blasting Powders and purchase 30 Runecloth.
Create 10 Dense Dynamites using the Blasting Powders and Runecloth. This should level you to 260, but if it doesn't, create a few more after you gather more of the ingredients. As an alternative, you can create Dense Blasting Powders to reach 260, but they turn green (require more creating to achieve a single point) at 255, whereas Dense Dynamite turns green at 260.
Purchase 45 Thorium Bars, or smelt them yourself if your second profession is Mining, then purchase 15 Runecloth.
Create 15 Thorium Widgets using the Thorium Bars and Runecloth. This will take you to level 275.
Locate an Engineering trainer and train as a Master Engineer. This opens up several more training options to continue leveling. You can also continue creating Thorium Widgets until they turn green at level 290.