Step 1
Pick a Primary and Secondary Monster type (dark, earth, wind, etc)
There�s one thing you have to remember when choosing a Primary and Secondary Monster type. Some things just don�t mix. You should apply this principle when constructing your deck. Dark and light will never mix, they will balance (cancel) each other out, and therefore you have a deck that has no purpose.
Fire is a destructive force, it fits in with Dark cards, but not Light cards, which should be more focused on positive things. Earth and Wind can generally be mixed with any of the other deck types.
Step 2
Choose approximately 18-25 monster cards, with 60% to 75% of the Monster Cards being of your Primary Type, and 25% to 40% being of the Secondary type. Your secondary cards should relate to your Primary type cards.
Try to add cards because you like them, the way they look, but try not to add cards that are weak.
Step 3
Choose between 25-32 Magic/Trap cards. These cards should also relate to the overall "feel" of your deck. Darker feeling magic cards shouldn't be in a Light or Water deck, and Lighter feeling magic cards shouldn't be in a Dark or Fire deck.
The magic/trap cards should be used responsibly. Cards like Raikegi, Monster Reborn, Change of Heart, and Dark Hole are all very strong cards, however, you should not base your deck around them. A Theme Deck is based on its monsters. If you rely on these cards, your deck is no longer your own, it is just like everyone else�s who relies on these cards. An example would be... The Beatdown deck. They all use the same magic cards, and very often the same monsters. They may have other cards to distinguish them from other beat down decks, but you can always tell that they rely on those key magic cards and monsters.
Step 4
Look over and review your deck
Remember in a themed deck, all of the cards must be connected and have to support each other. If any of the cards in your deck do not fit in with the theme you have given it, take them out and replace them.
Do not be held back by what people tell you that you "Need", use the cards that fit your theme.
Once you have successfully completed these 4 steps, you have a deck with a theme, which should be original, since those cards you chose were based on your personal preference, not popular opinion.