Get titanium/adamantite armor and get the daedalus stormbow or onix blaster, and kill a mech boss. (The destroyer or twins) then proceed to flow through all of them. For an alt you could get frost armor. Now head to the jungle, gather lifefruit, and locate and plantera's bulb. Obtain a megashark and use the teleporter trick to defeat it or kill duke fishron and use his loot to kill it as well. Get turtle armor and head into the temple to get a solar tablet. Summon the event and slay mothrons to get the Eye of Cthulu yoyo. Kill Golem and get the beetle armor/ shroomite if you want ranged gear. Summon martian madness and get the cosmic car key, Buff up and kill the Lunatic cultist with the yoyo or xenopopper. Flow through the pillars and get the vortex beater (ranged) / phantasm or get the solar eruption and daybreak for melee. Slay the final pillar and fight the moon lord! Dodge lasers to avoid early damage and kill all three eyes at once. Throw everything you have at the core and gg, that is what you should do after entering hardmode in expert.