I would first off ask yourself, 'How do I plan on winning with this deck?'. From there it is easier to remove pieces that do not impact the board in the way you want. If you are planning on blowing your opponent out with Overrun in mono-green, then cutting
walls and low-impact artifacts might be a start. When you unlock more cards for the mono-green deck, start cutting out the less impactful creatures and keep the more relevant ones (with usefull abilities or high power/toughness). Ramping also helps.
Ideally the cards you want are the cards that are 'playable' at any point in the game. Some cards are so narrow that you are much better off removing them to make your deck more consistent. Ask yourself if you would be happy to draw this card at most or
all points in a game. For example, if your opening hand has a Doom Blade, you're pretty happy. If the board stalls, drawing it will help you gain advantage. If you are losing, it will certainly help slow your opponent down and give you a chance to catch
up. The only time it is a dead card is when your opponent does not have a single creature on the board, and well, most of the time, you are winning that game anyway! Now compare that card to Wurms Tooth (terrible comparison, but a lot of people run them).
This card is very narrow, and will not dig you out of a bad spot.
Also, try and always play with 60 card decks. Anything more than that and you are hurting your chances of drawing into your winning board state.
Pay attention to your mana curve; 36 cheap spells might seem like a good idea, but your deck will likely lack the reach to close an opponent out and 36 expensive spells means you will almost always lose before casting enough relevant spells to take over.
Find a blend that suits your style and run with it.
Editing the decks is one of the core gameplay mechanics of Magic: The Gathering. It helps you make the deck your own. Maybe you want more mana ramp in the green deck to get your bigger creatures out faster. Maybe you want less creatures and more burn spells
in your red deck. It's all about you and how you want to play your game. Of course you also may end up editing a deck to deal with one of your opponent's decks.
why would you edit your deck? well you need to look at it like this, treat your deck as your family its good to have lots of people to see but you are unlikley to see the one you really love that often. a good size deck would be around 60 cards this reduces
the chance of pulling the cards you dont want and can speed up your attack dramatically make sure you have an even mana build up so you can play spells often rather than all big creatures.
There are several reasons why you would want to edit your decks. For one, you can only have up to 100 cards in a deck, so once you unlock all of the cards available for one there will be more than that to choose from. For example, in the Dream Puppets deck
I removed some of the cards that I don't like such as Archive Trap and Wheel and Deal to make room for the ones I do like. Another reason to edit your deck is to streamline it to be more efficient for the purposes you are using it for. If you remove the cards
you don't want to use it makes the chances of pulling a good card a little better. As to how would you edit them...you have to go into the Deck Manager and select the deck you want to edit, then push A on a card you want to remove or add. Since you have limited
customization abilities with your decks, I do very little editing to them unless I am taking out the cards I absolutely have drawing into my hand.