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 referred to as card salad, and you will be not 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
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.
Control quantity - You should have:
LV 1-4: Around 12
LV 5-6: Around 4
LV 7-8: Around 1 or 2
LV 9 and above: Never more than 2, depending on the deck. Some decks can and need to be all high monsters. In fact Malefics are an Arcanetype with only high level monsters with the two main ones being level 10. They are easy to summon needing only the nonmalefic forms, and a field spell.
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 - 10 traps, no more, no less, for any deck type. The only exception is for decks that rely 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, Dimensional Prison, and Bottomless Trap Hole
If you're running a large monster count, such as in Dragon Rulers or Mermails, 3-6 traps should be enough. A good choice for a deck that relies solely on monsters would be Royal Decree.
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.
Go get the cards - Well, next you have to find the cards you need. Always go to the local card shops and ask around for trading deals, or singles.
Get playing with friends and local players for the fun and to find out more about the deck, like in order to find it's weaknesses.
After a few more 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 gameplay.
Half your deck should be made of monster cards. One monster card for every spell/trap in your deck. This is a good starting point. You'll most likely end up with a few more monsters than 20.
Choose your decks. Structure decks are good for cards that work together and of the same type, but they don't have synchro or tuner monsters. Starter decks have lots of different kinds of cards and tuner/synchro cards. Booster packs have cards that don't work together, so get them at your local store, because when you buy them at the store, on the back of the package, it will tell you what variety of cards it is giving you. The other option are websites that sell every pack from the first to the latest, meaning you'll find everything. Also they sell singles of every card. This allows you to get cards that might be extraordinarily rare. It's not always cheap but if there's a card you absolutely need, it works
You should have around 13 level 4 or below monsters. They come in three main types- Normal Monster, Effect Monster, and Tuner monster. Balance each type so there are about more Normal and Effect Monster cards(like 8 or 9) to about 4-5 Tuner monsters.
Normal, Yellow bordered non effect monster cards without tributing should have 1600 or more attack. In the current metagame (the popular cards and decks that people use), the only reason people run a normal monster is so they have a target for Rescue Rabbit, granting them a free rank 4 xyz.
Effect monsters attack can vary, but make sure their effect helps you. Flip effect monsters can have very good effects, such as inflicting damage to your opponent based on specific types of cards your opponent has on the field, or destroying monsters, such as Old Vindictive Magician. The good attack for a level-4 or below effect monster is 1600 or more (unless it's a flip monster, their attack and difference does not matter) If they can attack directly, or let another monster gain a large amount of attack, it does not matter what attack/defense points they have. Some cannot be destroyed if the monster has a certain amount of attack, good for defense position monsters, and some can attack/defend more than once. These are great monsters. Shield Wing, which has only 900 defense, can defend twice per turn and not get destroyed.
Cards with one tribute need at least 2300 attack. Have 4 or less of these. It depends on the card and deck. Some Arcanetypes, as well non-Arcanetype based cards, are one tribute level fives or six with lower than that attack and can be extremely useful. For instance Reeze Whirlwind of Gusto has only 1900 ATK, and yet she is the best Gusto after Sphreez, because she switches a card on your side of the field with an opponent's monster, and if she dies you still keep the monster you swapped with.
Cards with two tributes need at least 2600 attack. Fill in the rest of your monster card section of your deck with these cards.
Trap cards are useful, but especially the ones that prevent attacks from enemy monsters. You should have about 6 or more of these trap cards, and some examples are Scrap Iron Scarecrow, Negate Attack and Sakurestu Armor. Spellbinding Circle and Nightmare's Wheel prevent the monster from attacking as long as it is on the field. Your style matters though, it's really up to how you play.
You should have ten trap cards, and so pick four others that will help you in game play too.
High level monster out. Even one tributes are near impossible if you only normal summon to win, because this from of summoning is a once per turn occurrences. So you can get your low level out and it will most likely die before your next turn, thus you'll need to summon another and at that point your pretty much screwed.
There is also a kind called Quick-Play Spell cards. They have a lightning bolt icon next to the word "Spell Card," printed on the card. You should have three to five of them, because they can be used on your opponent's turn. Book of Moon is a common Quick Play spell.
Refrain from using cards that are forbidden. Some examples are Graceful Charity and Pot of Greed. These cards are known to be too strong which would make your deck sort of 'cheating'. These cards can also make fights with other duelists.
Remember to never use forbidden cards in tournaments. You could use them when dueling with a friend but he/she might not accept the offer.
Test Your deck against a hard adversary, then make your deck stronger by buying more cards and don't buy unless needed.