Moves
Dark Void is hands down the best move in the format. The ability to put both targets to sleep with a move that is relatively accurate is great utility. This constant threat of putting both of your opponent's Pokemon to sleep is the closest thing Smeargle has to offensive presence, and it is a great tool for Smeargle to use. This in conjunction with Follow Me makes Smeargle really disruptive to the opposing team; with the ability to redirect attacks away from its partner, it can aid setup or give a free turn to attack. Follow Me is a rare move in the format due to its poor distribution, with the only other Pokemon with access to the move being Lucario, Furret, and Pachirisu. This gives Smeargle a niche in being the only attack redirector that can put its targets to sleep without being ignored by Grass-types and Safety Goggles Rotom-H.
Your choice for what to use in the last two moveslots comes down to the needs of your team. Fake Out is useful to flinch, immobilizing the target Pokemon and giving Smeargle another way of providing a free turn for its partner, as well as breaking Focus Sashes. However, Encore is a great way to lock opposing Pokemon into setup moves, letting Smeargle check Pokemon like Substitute Aegislash, Swords Dance Scizor, and Dragon Dance variants of Gyarados and Tyranitar. Additionally, it can be used to lock slower Pokemon into Protect, forcing them to switch out and waste a valuable turn in the fast-paced metagame. Spiky Shield is literally a better Protect, blocking all moves, getting chip damage when Smeargle is contacted, and being nigh impossible to lock with Imprison because of its low distribution. However, King's Shield lets Smeargle check physical attackers that commonly use contact moves, such as Kangaskhan, Garchomp, and Talonflame, as well as physically oriented Fake Out users like Mienshao and the aforementioned Kangaskhan. This, however, comes at the cost of the ability to block status moves, most notably Taunt. Also, it is important to note that Bisharp isn't affected by the Attack drop due to Defiant.
Set Details
Maximum Speed investment with a Timid nature is chosen to maximize Smeargle's Speed while minimizing the damage Smeargle takes from Foul Play. Affecting Fake Out's power isn't important due to its low power regardless. The rest of the EVs are allocated in such a way as to maximize Smeargle's overall bulk. Focus Sash is the item of choice due to Smeargle's nonexistent bulk making the use of any other item unfeasible. Moody is the ability of choice because it can very easily make Smeargle a monster to face in just a few turns. However, it is a double-edged sword, as Smeargle can get accuracy, evasion, and Speed drops. Own Tempo could be used if you don't like this unpredictability, but confusion-inducing moves are incredibly rare, and the chance to make Smeargle monstrous is almost always worth the tradeoff.
Usage Tips
Spamming Dark Void is counterproductive because it fails when used on already sleeping Pokemon, and the threat that Smeargle could use it creates mindgames that are removed if it only uses said move. Follow Me needs to be used with the same level of care as Dark Void, as the combination of said moves is what makes Smeargle so disruptive. Smeargle's support options allow it to give its partners free turns to set up or a free turn, so make the most of all the options Smeargle has. Finally, don't be afraid to use Spiky Shield if you are unsure about what to do. It is a great way to scout and see what your opponent's strategy is, and protection is all the more valuable on a Pokemon as frail as Smeargle. Spiky Shield also adds to the mindgames that Smeargle can play.
Team Options
Smeargle is a valuable asset to more offensively oriented teams. However, it also needs support to come in safely due to its immense frailty, making Pokemon that are solid switch-ins or switch-outs such as Amoonguss, Garchomp, and Talonflame solid teammates. Smeargle's middling Speed stat means that it benefits greatly from Tailwind support, making users such as Aerodactyl, Noivern, and the aforementioned Talonflame solid partners. Common users of setup moves, such as Tyranitar, Scizor, and the previously mentioned Aegislash, benefit from Follow Me, Fake Out, Dark Void, Encore, and King's Shield aiding their setup. Similarly, frail attackers like Mega Lucario and Mega Aerodactyl benefit from Smeargle's ability to redirect attacks away from them.
As for support that Smeargle itself benefits from, users of Quick Guard, such as Mienshao and Talonflame, are great partners to prevent the use of moves like Fake Out, Sucker Punch, and Gale Wings Brave Bird; similarly, Wide Guard from Pokemon like Aegislash and Machamp makes the mindgames that Smeargle can already create even more potent, and allows it to be less reliant on Follow Me. Pokemon with access to Drought or Drizzle, i.e. Politoed and Mega Charizard Y, respectively, are great partners for their ability to clear hail and sandstorm. Aromatisse's Aroma Veil is a great way of preventing Smeargle from being on the wrong end of a Taunt. Finally, other users of Fake Out provide lots of opportunities for Smeargle to use Dark Void, making users such as Mega Kangaskhan, Mienshao, and Mega Blastoise great allies, with the latter also benefiting from the Follow Me support keeping Water Spout at maximum power. This opportunity creation concept also applies to Amoonguss as it can redirect attacks away from Smeargle to fire off a Dark Void, as well as helping to preserve Smeargle's Focus Sash.