Decide your playing style - This is important, as it determines how people would look at you as a duelist. Are you a rash duelist who would rapidly summon, attack and activate cards? Or are you the deep thinker who would analyze your hand and field before making a move? Or maybe the duelist that would remove cards from play so your opponent cannot use them again? Considering these would help you choose a deck archetype.
Choose your deck type -a themed deck, or one that follows a specific theme of cards. NEVER have a deck with too many cards in it. This is refered to as card salad, and you will be drawing parts of combos.
Choose your deck basis - I'd rather recommend a themed deck, as the cards are least likely to get banned, resulting in sure playablity. Refer to the tips for a link about archetypes.
Choose your monsters - Once you know what deck you want to play, and how you want to play it, choose your base monsters. Every archetype will have at least 10 main monsters. Once you've put them into a LIST of cards you'll want, add in the appropriate support. Keep that to AROUND 18 cards, unless you REALLY need it, like in the case of maybe the Elemental Heroes.
Quantity Of Cards
LV 1-4: Around 12
LV 5-6: Around 4
LV 7-8: Around 1 or 2
LV 9 and above: Never more than 3
Choose your spells - Once again, around 1/3 of the 12 spells in your deck would be for monster support/combos. The rest would be attributed to favourites and staples. Add them into your list once you've decided.
Choose your traps - I'm very particular about this, 10 traps, no more, no less, for any deck type. The only exception is for decks HIGHLY RELIANT on traps, like decks which have many weak monsters. Among these, 3-5 should be support for your deck type, and the rest should be staples like Mirror Force, Sakuretsu Armor, and Trap Hole.
Lay out your cards and make sure that they work together well. Its no use if you have cards that do not work together well. List the cards that you need to improve your deck and make it your business to buy those cards. Watch the cards your common opponent plays. Also add some generic cards in your side deck that you can use later on, in between duels.
After a few games, it's time to cover your deck's weaknesses and exploit it's strengths, and modify it with new support cards. During this time, you should already have taken out at least 5 cards, and replaced them with something more useful.
Then, repeat the process of playing and modifying, and never be afraid to have a perfect deck. A deck should be everchanging, with new cards every week of game play.~Remember to keep as close to 40 cards as you can.