When you design a light, first you have to understand you need to limit the size of your light design.
For example I have limited my light housing to be 15cm x 15 cm x 15cm.
This is limited by the 3D printer's hight but also helps to design with a solid dimensional foundation,
no wasting time on designs that will have to be either cut or reshaped later .
Sketch ideas manually or you could just start on the computer, I use Rhinoceros to design my lights.
Step 1: 3D Printing Your Light Design.
Try not to make something too complicated first.
Take your time and design simple shapes to understand 3D printers potentials and limits. It also help you greatly to understand the actual print size and that designed shape in 3D is different from what you see on your screen.
Depending on the printers and material and types of prints the design can vary a little. For example I use a Makerbot rep2 and it uses PLA plastic.
If I go in to the details depending of the colour of the plastic it is preferable to adjust the melting temperature.
I find 1.3 mm wall thickness a great combination of strength, lightness and transparency.
Remember to make your light fixtures with standard bulbs sizes in mind.
(or you would be struggling to look for bulbs that would fit the funny shapes )
I used E27 base light bulb kit so it fits perfectly together.
Think carefully about the distance between the light fixture and the bulb, it might melt depending on the bulb type and plastic type. Some LEDs produce a lot of heat so be careful not to melt anything.
(usually PLA melts like butter. Be extra careful )
Step 2: 3D Printing Lights Tips
If you are using a PLA plastic FDM printer you can change colour by manually feeding your machine during prints. make sure the printer is hot enough to resume printing if not it will not deposit the plastic filament firmly and ruining the print.
For safety leave some space for the bulb to breath in your design even the bulbs are in LED.
In all my design I have holes where the bulb is held together so the heat dissipates out of the lighting housing.
Think how you will use the lights. Think about how you would decorate the house with it. Don't just design pink harts of other kitchy shapes.