Just search "game name loadiine" and you'll find most games in a format that's ready for Loadiine use. Then you place the code, content and meta folders in SD:\wiiu\games\game name [game id]\
The game ID you can find on
http://gametdb.com
If you don't have the game ID and can't find it, you can use any 6 characters, but covers will not show up. If you don't use cover view mode it doesn't matter.
Some games will be missing the meta folder, which means that the icon won't display in Loadiine. You can either get this folder from an update if there is one (read further below for details on that) or get it from the WUD (read below)
WUD files:
If the game you want is not available in loadiine format, you can extract the WUD yourself.
For that you will need Uwizard:
https://gbatemp.net/threads/uwizard-all-in-one-wii-u-pc-program.386508/
For Uwizard to work, you will need the Wii U common key, Wii U Espresso Ancast Key and Wii U Starbuck Ancast Key. These are copyrighted so I can't provide them but they should be easy to find on Google. Then you just paste them into the settings tab in Uwizard and they'll be saved for future use.
You'll also need the game's own title key, this is usually included in the .nfo file in the download but is sometimes found on the page you downloaded the game from. If it's not in either of those places then a Google search should help.
Once you have all that you simply open the WUD image in Uwizard, paste the title key in the box, and click Extract Game Files. It will create code,content and meta folders that are ready for Loadiine use, just place them in a correctly named folder on the SD. If you don't see the code, content and meta folders look inside the data folder.
Game updates:
Some games have updates for them, and Loadiine is not able to load updates installed on the Wii U yet. So for updates, I use NEW NUSGrabber GUI:
https://gbatemp.net/threads/download-the-new-nusgrabbergui.411228/
Simply go to the settings, set the correct region and enable "Automatically decrypt
downloads". Close the settings and hit "Update VersionList". It will take a while but once it's done it should list every game of your chosen region along with all their updates. Just click Download and the update will be downloaded and decrypted automatically into a subfolder in the same folder as the program.
The first time you download something with NEW NUSGrabber GUI you will have to point it to the ckey.bin file, this is the same Wii U common key mentioned previously, but you will have to use a hex editor to paste in the common key as hex and then save it as ckey.bin. The resulting file should be 16 bytes.
To use the update in Loadiine, create a folder inside SD:\wiiu\games\game name [gameID] named updates. Inside that folder create a folder named vXX (replace XX with the update version you downloaded) and paste the code, content and meta folders from the update inside that folder so you end up with SD:\wiiu\games\game name [gameID]\updates\vXX\ containing code,content,meta
Then make sure that you select the correct update folder when launching the game from Loadiine.
There you have it, a mostly complete guide on how to set up games for Loadiine