This melding exploit in Monster Hunter World lets you get the best decorations every time you meld. The melding process at the elder melder is completely random for the most part. You give him decorations you don’t need and in return you get a random decoration in return. Tier 3 decorations are the most valuable and also the rarest to receive so it can take a long time until you get the one you really want. Here is how you can get any decoration you want from the melder.
Save your game to the cloud storage before you meld and restart the game. This will keep your current progress and you won’t lose any items you are about to meld. Go back to the elder melder and meld strange streams in the First Wyverian Ritual. Meld sets of 3 as often as you like and write down what you got out of each roll in slot 1-3. Once you melded a couple of times and found decorations you want, load your previous save game from the cloud and restart the game again.
The rolls will have the exact same result as before so you know now what each slot will give you. If you found something you like in roll 2 for example, start a quest and complete it. This will skip one roll and you can now meld the item you want.
Also check out this reddit post from joaquinnacpil. He posted an awesome spreadsheet that lets you track your decoration cycles so you know exactly when and what you get.