I have some extremely simple deckbuilding advice here.
When building a deck one should start by thinking up one's win condition or the goal of the deck. For example a Quasar deck's win condition is to bring out Quasar. Pretty straightforward.
The next step is to add all cards necessary for your win condition to the deck. Now you add as many of the staples to your deck as you can fit. There are staples for almost the entire format, like monster reborn, and MST, (just examples, there are more) and there are staples for the type of deck you are using, like the zombie staple, Zombie Master.
Next is support. When you are adding support to your deck, you are adding cards that help pull off your win condition more easily. There is a huge variety of support for ANY type of deck. So in this stage you are adding any cards in your deck that will help you pull off your win condition. At this point the size of your deck is not important, so add whatever cards you think that you will need.
This is the point where you lower your deck to forty cards, or as close as you can get. The reason for this is you chances of drawing a certain card you need in a forty card deck is much higher than drawing that card in a sixty card deck. Continueing on this, and getting a bit off topic, this is a good time to add multiples of your best cards, as drawing a card you have three copies of is much more likely than drawing a ard that you only have one copy of. Now, back on topic, you want to eliminate any cards in your deck that seem like they are too situational, or are not very useful now that you have seen all the cards you plan to include in the deck.
Remember not to stick too closely to this paragraph as certain decks are better off with more cards, however I am not getting into that now.
This paragraph, just like the one above, is not true in certain decks, however for most decks both of these paragraphs should be followed. This step is simple. In this step, edit your deck until your spell/trap card to monster card ratio is as close to 1:1 as possible.
This is normally where I would talk about side-deck building, however there are a huge amount of people who can explain this better than I, so I,ll leave it to the experts.
This final sttep should be repeated over and over untill your deck isn't improving anymore, the it should be repeated some more. Seriously. This step is playtesting. You must test how your deck runs, and pay close attention to any flaws you can find, write them down even. Then go back and edit your deck according to it's flaws. Repeat this step as many times as possible.
I am not the most skilled person at guides and there are liky many flaws to this. Post qny questions here, or PM me.