Growing Your Collection
Since the cards are in your collection determine what tools are available to you, it's important to go about it the right way and not squander your resources. This is true whether you're a new player, a free-to-play player, or a player who plans on spending money on the game.
Generally, the main ways to grow your collection are via the daily missions, drafting, the achievements, the end of season rewards, and from gaining gold every 3 wins. You also gain 2 gold for the first win of the day. For the most part, you can play the game as you see fit and continue to gain achievements for quite a long time (these achievements scale up in the rewards you gain as you go through the higher levels). Your main focus in gaining rewards should be to reach Field Marshall (FM) at the end of the season with each nation, and to complete your daily missions. This will ensure you efficiently maximize the gold and packs you receive for the time spent in the game.
When it comes to your gold, you may choose to either spend them in packs or to draft. It is completely possible to collect the cards you need to play this game competitively without ever drafting. It's worth noting that your average game in draft does take longer due to the fact that people are generally playing unfocused decks. If you're primarily interested in jumping straight into constructed and don't particularly have a lot of time to play, it's perfectly fine to to spend the gold on packs.
That being said, if you're good at drafting it could very well be worth the effort (in terms of the rewards gained). Due to the 15 gold entry cost and the fact that you have to win around 50% of your matches to roughly break even, I would suggest holding off on playing draft until you become more familiarized with the different cards in the game. If you do plan on drafting eventually, however, it can be worth saving your gold for more runs instead of packs. If you plan on playing kards a lot, spending more time in draft makes the most sense to maximize your rewards (assuming you're winning your games, of course).
As you finish your seasons and as your collection grows, you'll undoubtedly receive many gold cards. Unless your collection is mostly completed, I would recommend recycling all of them for resources. Since you gain resources equivalent to their crafting cost, you don't lose anything; you can always recraft a card for what you gained, if needed. The resources you gain from golden cards are incredibly useful in acquiring the cards you need for your decks. The exception to this would be if the gold card is card you were going to craft anyway.
What You Should Aim to Craft
In regards to rarity, you should aim to craft the limited cards you need first. There are currently around 127 limited cards in the game (in the future there are bound to be more released), and you can add up to 3 of a type in a deck. Since you gain around 1 limited card per pack, this means you'd need to purchase at least 381 packs to have a chance of acquiring them all. Considering this very large pool, there's no guarantee that you'll gain the limited cards you need. It would take a very long time to do so via packs alone. You should avoid crafting standard cards, since you often get 4 in a pack and will therefore acquire them rather quickly.
I'd also avoid crafting cards from the minor nations until you're satisfied with your cards from the major nations, as the rewards from the end of the season and daily missions are tied to the major nations.
As for crafting, your general priority should be as follows:
1) Commonly played removal orders of limited rarity. These cards are incredibly versatile, and can be played in any type of deck. The reason why removal is the top priority is due to the fact that there are many cards in the game that put you in a severe disadvantage or make you immediately lose if you can't deal with them. Generally removal that can kill units are better to craft early on, but even cards with bounce effects can be worth crafting at this stage if they're commonly played in the meta. You should aim to do this for every major nation.
2) Commonly played 1 and 2 kredit openers of limited rarity. I consider an opener as a card that you can always play on turn 1 and turn 2 while going first, regardless of what your opponent has (or doesn't have) on the board. Every deck needs something to play in the early game, and often times these cards can overlap from deck to deck. This is the second priority after removal, since not playing anything in your first 2 turns can lead to severely falling behind in the match.
You should do this for every major nation.
3) Other commonly played cards of limited rarity. There are many limited rarity cards (that don't belong in the above categories) for every major nation that are considered staples for multiple archetypes. For example, this could be a 3k unit that's so powerful, it frequently makes an appearance in multiple deck types.
4) Good limited rarity cards for your played archetypes. Once you have the staples, I'd recommend homing in on the cheapest competitive archetypes for each nation, and crafting the required limited cards for them. This is so that you can more easily achieve Field Marshal with them every season.
5) Cards of special rarity to bolster your chances for FM. If you're skilled enough, it is possible to achieve FM with every nation while only playing decks consisting of limited and standard rarity cards. However, this may prove to be difficult with certain nations. Since getting FM with every nation is a top priority, acquiring the special cards that are used in the archetype you're struggling with could be beneficial.
Once you reach a point where you can confidently reach FM with each nation every season, how you grow your collection is a matter of what you prioritize in the game.
If your goal is to play competitively on the ladder, I would recommend crafting all of the cards for a single archetype that appeals to you (starting with the specials first, and then moving on to the elites). You'd prioritize your crafting based on what would improve the deck the most. You should then proceed to do this for each archetype you're interested in, working on a single one at a time.
If you're mainly interested in having variety and playing many archetypes, I'd instead focus on crafting cards of special rarity first (prioritizing them in the same way you did the limited cards). Once you craft the special rarity cards you need, you'd then prioritize crafting the elite cards based on how powerful and commonly played they are in the meta.
Of course, when trying to decide what to craft, it helps immensely to have your specific decks in mind ahead of time. In order to do that, it helps to have good deck building skills. We will be covering deck building, and what makes a good deck, in the next section of the guide.