Deckbuilding in one sentence.

Understand and support your win condition while stopping the opponent from doing the same.

Understand your win condition.

The spark of inspiration for any competitive deck boils down to the same question: how do I win the game?
In the Pokémon TCG, there are two main win conditions.

Take 6 Prize Cards

Use a combination of a primary and secondary attackers to knock out the opponent's Pokémon and take 6 prize cards. Sometimes, you'll win early if the opponent has no Pokémon remaining in play.

Deck Out

Use cards that directly reduce the opponent's deck size, or, more frequently, use a combination of energy-denying cards like Crushing Hammer and retreat-locking cards like Block Snorlax to trap the opponent out of the game.

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Regardless of what route you choose, the start of your list should include your primary attacker, any Pokémon that directly enable them, and the Energy they need to attack.

For example, a Mega Clefable ex deck may start with 4 copies of itself, an Aromatisse line to search out the Energy that gets discarded, and 12 Psychic Energy.
In contrast, a deck like Crustle (DRI 12) may start with no other Pokémon accompanying it.
Sometimes, a deck may also be built around multiple attackers at once. For example, Clefairy ex + Teal Mask Ogerpon ex complement each other to hit several other decks for Weakness.

Support your win condition.

Once you have a plan to win the game, the majority of your deck should go towards supporting that win condition. Cards that search out Pokémon, accelerate Energy, and draw more cards round out a list.

Deny the opponent

Even if you aren't playing a Control deck, denying your opponent resources will round out your list. Cards like Judge, Boss's Orders, and Jamming Tower force the opponent into uncomfortable positions. Most decks will run some combination of these versatile support cards; the weaker your deck is in the metagame, the more you'll rely on niche effects to target bad matchups.

Modifying a Deck

Once you've prototyped the deck, you're ready to start playing games and refining the list. Key factors include:
(1) Can you consistently set up your Pokémon? If not, add Items like Ultra Ball to find them or Supporters like Lillie's Determination to draw more cards.
(2) Are you struggling to handle hand disruption? Include on-board draw engines like Fezandipiti ex, Dudunsparce, or Drakloak.
(3) Is a specific matchup giving you trouble? TPC often prints "silver bullets" meant to counter dominant strategies. Common examples include damage prevention (Shaymin, Battle Cage), ability negation (Garbodor, Team Rocket's Watchtower), and splashable OHKOs (Drapion V, Lillie's Clefairy ex).

Deckbuilding Example 1: Team Rocket's Honchkrow

In this example, I've decided I want my win condition to be taking prizes with Team Rocket's Honchkrow. My deck list starts with a 4-4 Honchkrow line, 4 Team Rocket's Energy, 4 Ignition Energy, and the 20 Team Rocket Supporters discarded for Honchkrow's attack.

Now that the win condition is established, we support it. 4 Poké Pad and 4 Ultra Ball provides a way to find Honchkrow, while 4 Team Rocket's Transceiver and 4 Roto Stick find Supporters. 4 Team Rocket's Factory and a 2-1-1 Team Rocket's Porygon-Z line provide additional card draw.

With this, a simple 60 card list is complete! Our Supporter lineup here includes Team Rocket's Archer and Giovanni, which serve double duty as methods to disrupt the opponent. You're ready to start testing and making changes.

If we compare the list to the Top 8 list from Champions League Fukuoka, we'll see a few potential changes. Ultra Ball and Roto Stick were cut to make way for additional tech cards. Team Rocket's Articuno prevents Phantom Dive from KOing a benched Porygon and a variety of other effects from other decks. Fezandipiti ex provides a way to draw additional cards after Unfair Stamp. Night Stretcher importantly allows reusing Honchkrow or KO'd Fezandipiti, and Miracle Headset is a simple choice for ACE Spec to add more Supporters to hand.

Deckbuilding Example 2: Mega Clefable ex

This time, I've decided I want to use Mega Clefable ex to take Prize Cards instead. We'll start with a 4-4 Mega Clefable line, a 2-2 Aromatisse line to consistently add Energy from deck to hand, 12 Energy, and 2 Energy Recycler to place Energy back in the deck to be reused.

Once again, 4 Poké Pad and 4 Ultra Ball provide ways to find our Pokémon. Let's learn from our previous deckbuilding example and also include 4 Night Stretcher. This card has added value by taking back Energy from the discard! We'll also include 4 Wondrous Patch for Energy acceleration. To draw cards, 4 Lillie's Determination, 4 Judge, and 4 Mystery Garden will refresh our hand.

In this case, we're also missing a key element of supporting our win condition: damage. Unlike Honchkrow, Clefable caps out at 280 damage. Unfortunately, Mega Pokémon ex exceed that number. Let's include the Maximum Belt Ace Spec, 2 Kieran, and a Lillie's Clefairy ex as ways to increase our damage.

Finally, we round out the list with 2 Boss's Orders to disrupt the opponent and chase down low-hp Pokémon ex to finish the game.

Much like before, testing will likely reveal an abundance of consistency cards and a lack of meta-specific techs. Mega Clefable provides 3-prize cards, so including Jacinthe or Wally's Compassion may help slow down the opponent while a single-prize Clefable can use its Metronome attack to OHKO Mega Lucario ex, Teal Mask Ogerpon ex, or Dragapult & Raging Bolt ex (with Lillie's Clefairy in play).

Resources

Card Watch: A list of staple cards to consider when making a new deck.
pkmncards.com: a database of Pokémon cards with a robust Advanced Search function. Use this to find supporting cards you may have missed.
limitlesstcg.com tracks tournament results. Find the tested, verified decks professionals bring to events.
tcgplayer.com the largest Singles marketplace for US-based players. Use this to purchase cards for your deck.
cardmarket.com the largest Singles marketplace for EU-based players. Use this to purchase cards for your deck.