The instructions for all sets contain a “bill of materials” on the last pages which list all the parts required to build the model. Once you have these parts, simply follow the instructions to build the model.
If you do not yet have all the parts you need, you can order them online, for example from one of these services:
- The “Pick a Brick” service on the official LEGO website
- The “Buy Bricks” service, also on the official LEGO website (service section)
- BrickLink, an online marketplace for LEGO sets and individual bricks
All items available on BrickLink are offered by 3rd party sellers. Sets and bricks are available in new and used conditions. Bricks on BrickLink are usually, but not always, cheaper than via LEGO’s own services. Also, more parts are available there. However, some special parts may only be available directly at LEGO, and in certain cases some parts may be cheaper at LEGO. BrickLink also allows you to create brick lists and can then automatically find sellers offering the parts you need.
I recommend first trying to find parts on BrickLink, and then going to LEGO’s own services for individual parts that might not be easily available (or expensive) at BrickLink.
In addition to the building instructions in PDF form, I also offer my original Stud.io project files as downloads. Stud.io is BrickLink’s own editor application used to create custom sets and instructions. It can also automatically connect to your own BrickLink account and create wanted lists for the parts in a model. I very much recommend using this feature if you want to order parts from BrickLink.