You get however many Sparks are needed for your chosen weapon, 1 spark for one-hand or 2 sparks for two-hand.
You get the crests you need for the craft level you want, in this case 60 Gilded Ethereal Crests, and buy an Enchanted Gilded Ethereal Crest from the vendor either next to the Enchanting station or to the right of the crafting order building as you face it.
If you want specific stats, you go to the Auction House, enter Algari Missive into the search bar, and select the one that offers the stats you want.
If you want an embellishment, you go to the Auction House, then type the name of the embellishment reagent you want into the search bar and buy it.
You take all of the materials you need, seek out a crafter willing to make your item through the appropriate chat channels (Trade or Trade (Services)). When you find one, write their name down or copy it by right-clicking it in the chat and selecting Copy Character Name.
Finally, you speak to one of the Crafting Orders NPCs, find the item you wish to have crafted either by navigating through the drop-down menus on the left side of the panel or by typing the item’s name into the search box, take a moment to make sure you’ve put in everything you want (Missive, Enchanted Crest, Embellishment, et cetera), type or paste your crafter’s name into the name field in the upper right of the panel, select the minimum quality from the drop-down menu nearby the name field, enter the amount of gold you’ve decided to put toward their services, click Place Order and await your order’s completion.
If paying for it is an issue, then you can entirely bypass the haggling portion by just being your own crafter because you can send these same crafting orders to your alts to make them yourself.
If you decide to pursue a Public Order, then expect to receive a lower quality craft regardless of the price you set. Public Orders have no quality assurance as Guild Orders and Personal Orders do.