The Pentacle is a representation of the Universe as a whole, omne in parvo (Latin for "all in small"). First you need to make a symbol that, to you, represents the everything in the universe. This includes prom night, Ethiopia, and airplanes. From Aleister Crowley's Book 4:

"There is, therefore, nothing movable or immovable under the whole firmament of heaven which is not included in this pantacle, though it be but 'eight inches in diameter, and in thickness half an inch.' Fire is not matter at all; water is a combination of elements; air almost entirely a mixture of elements; earth contains all both in admixture and in combination. So must it be with this Pantacle, the symbol of earth."

To help with the symbol make a list of everything in the universe. After you have completed this list, look at everything, and combine them into smaller groups. Ex: trees, grass, and dirt, would all go into the same grouping of earth. After you have minimized your list as much as you feel is possible find a symbol for each thing left on your list. You can use traditional symbols, or even make up your own.

Next you need to decide what material you want to use. Traditionally natural materials were used. However, there are so many wonderful man made materials out there for building today, that personally I don't see any problem using them. Again, do what feels comfortable for you. The diameter of your pentacle should be between 6 and 8 inches.

Once you have your symbols, material and size you want to use, you need to lay out the design of your pentacle. You can keep it simple, or embellish it a bit. Make sure you incorporate all the symbols that you designed earlier. When you have drawn out your design, go ahead and get your materials together and construct it.

Below is one of the first instructions on how to build a pentacle.

Take pure wax, or a plate of gold, silver-gilt or Electrum Magicum. The diameter shall be eight inches, and the thickness half an inch.
Let the Neophyte by his understanding and ingenium devise a symbol to represent the Universe.
Let his Zelator approve thereof.
Let the Neophyte engrave the same upon his plate, with his own hand and weapon.
Let it when finished be consecrated as he hath skill to perform, and kept wrapped in silk of emerald green.