Planning
Nothing is going to be done well unless you have a decent plan to go by. Even if it is just a few rough sketches and dimensions, a plan is a valuable visual aid and a target to stick to. It helps make sure that your parts are the same size and shape, and helps you work out where you need to start. If you are internet savvy, you can search on-line for templates, which detail every part you'll need to make your tank, walking death-machine or flying contraption (I'm not going to share any for copyright reasons, but trust me, they exist).
When making your own plan and whilst working, it's a very good idea to write down as much as possible. Write down dimensions, draw the model from the front, side, rear and top. Use reference pictures and images on your computer. All of this will help you through the process, and alert you of any changes you may need to make.
Tools
Tools are essential if you want to scratch-build. Whilst a sharp craft knife is the utter essential, it's best to invest in other items too. A straight ruler (preferably metal) is fantastic for getting your lines straight and perfect; essential for tank building. A drill is great for pinning together pieces, or adding details such as pitting and bullet holes.
Another neat tool I have is a card circle cutter. This device is a godsend.. It's very handy in cutting out circles and curves, and despite the name, works very well on plastic.
Materials
Even a novice will tell you that you need some materials to get started. Plasticard is key here. It's easy to work with, strong, and takes paint well. for a good build, you'll need a wide variety of sheets, tubes, rods and strips. Thinner plasticard is great for detailing work and curves surfaces, whilst the thick stuff is ideal for basic blocking and structure. You may want to look into textured plasticard to add just a little bit of extra detail. Tubes and rods are well suited for making gun barrels, fuel tanks and exhausts. Strips aren't essential, but I find them very handy in making trim and edging on armoured panels.
Wires, chains and cables are another valuable asset. You can often find rolls of chains and hose cabling in model shops, and whilst it isn't always cheap, it does last a long time and will greatly improve the end result.
Donors
Some structures and complex shapes, such as plane fuselages, energy guns and crew, are too complex for a starter scratch-builder, and difficult even for more advanced modellers. Alongside the main build, I often recommend that you find a donor tank or model to rip parts off. Iconic and intricate parts such as tracks, Lascannons, crew members and turrets, can be ripped off other models and used in your creation, to add a bit of detail and make things more recognisable.
So with these tips, I hope to see more scratch-building. If you are working on something, or plan to, feel free to share it on the Facebook page.