Instructions
If you haven't already, create a new guild. You MUST be the Guild Master to create a tabard for a guild, otherwise you will just be playing dress-up.
Find the visitor's center in a major city.
Talk to the Guild Master.
Get as many of your guild members together in the Visitor's Center as you can.
Have everyone flip through the designs until they find something they like. Have one of your members record every design that someone finds interesting.
Ask your members to vote on the designs that were recorded as prospectives. Majority vote is the fairest way to choose a design for the guild.
Repeat steps 5 and 6 for the background color, border design and border color.
Once your guild has chosen all the elements of the tabard, talk to the Guild Master again, input the design elements and tell them to finalize it!
Buy a tabard! They usually cost 1 gold, but can be cheaper if you have a good reputation with the city in which you are buying.
Be aware that if you already have a guild tabard from a previous guild, it will have gone grey before now. In finalizing a tabard design for your new guild, your new design will simply take the place of the old one. In short: if you already have a tabard, you do not need to buy another one!
Help your younger guild mates buy their tabards! Have everyone fly their guild colors with pride!