First and foremost, consistency is key to making your deck function. It’s best to include all four copies of an important card for your deck or craft a few more copies of an ideal card, rather than including a bunch of one-offs that fill the space.
If you’re building a multi-faction deck, be sure to include some cards that will draw specific sigils (like Seek Power) or grant-specific colors of influence. Usually, some of the dual-faction power cards will be enough for a two-color deck, but Seek Power, the 2-cost Influence strangers, and some of the cards found in the Time faction are great ways to ensure that you get all the influence you need to play your cards.
Keep your deck to 75 cards if possible. It’s not the end of the world to go over, but your deck will typically perform less consistently the more cards you have in it. This may mean making a few painful cuts of cards you want in your deck or dropping down to 3 copies of less-important cards.
On that same note, enable advanced deckbuilding in the options menu. By default, the game adds power sigils automatically based on the cards in your deck. While this isn’t a bad thing, sometimes it can add more than your deck needs, and it also won’t use some of the optimal power cords for your deck (such as way stones, diplomatic seals, or banners, if you have them).
Sometimes, simple is best. When you’re starting, try to have your deck focus on one specific goal or theme.
Pay attention to how much power the various cards in your deck cost (see the chart shown on the deck builder!). While the minimum you can have is 1/3 of your deck, if you’re running a bunch of high-cost cards, you may need extra power to ensure you curve smoothly into the cards you’re trying to play. Trust me, having enough power (and influence) can make all the difference.
On that same note, think about your goal and compare it to the relative cost of cards in your deck. If you’re trying to play aggro, but most of your cards are 3-4 mana, you’re probably not going to do well. If you’re playing tempo or control, you should probably have a fairly even amount of cards at all ranges of mana. If you’re ramping into big creatures, you should have mostly high-cost cards, etc.