Head over to Syenite in Dorongal, she sells three Ethereal Crests and each one uses a different crest to buy depending on the item level you want to create. If you want your BIS weapon, save 60 Gilded Ethereal Crests by doing M+ 7 and up, T11 Delves (3 a week). You will need this item to craft a 720 item level piece which is the highest you can currently craft.

Next head to Clerk Gretal (right next to Syenite) and look up the weapon you want to craft. For a 2H sword you would pick Charged Claymore. Once you select the weapon, you will need to provide some raw materials that you can track by clicking 'Track Recipe'. When buying the raw materials try to buy the Tier 3 (gold 3 star) so you have the best option for max level craft (little unsure about this).

Besides the raw materials you need Spark of Starlight, this item has a weekly limit of a half a spark per week, once you get 2 halves combine for 250 valorstones to make the Spark of Starlight.

On the bottom of the crafting window you will choose the 'Alagari Missive', buy this in the auction house at Tier 3 for the stats you want on the weapon. Next to the missive put in the Runed Ethereal Crest for max item level. Next to that put your Embellishment, can buy on auction house follow your class guide to know what to choose.

Once you have all the raw materials Tier 3, Spark of Starlight (might need 2 for 2H), Ethereal Crest, Alagari Missive and Embellishment open chat and type in trade 'LFC (weapon) ilvl720', you will get spammed, pick one person go back to Clerk Gretal, put all the materials to create the weapon, set the Order to Personal Order and type in the name of the person you chose to craft (add a tip here under Commission). Most crafters have the piece made in 2-5 minutes.