Is your Wii U hacked? If not, follow
this guide.
Once you have Aroma and Tiramisu installed properly, install
NUSSpli as well and download the games you want with it. Due to Wii U's hardware limitations, WiFi download speed is limited to ~1.5MBps, so keep that in mind if you plan to queue a long list of games (make sure to disable standby functions before that so that your console doesn't go to sleep). If you have an original Wii USB LAN adapter, you can increase the maximum download speed to ~3MBps.
For the games to work, you'll need
latest sigpatches as well.
Download the
01_sigpatches.rpx file and paste it into
/wiiu/environments/tiramisu/modules/setup and
/wiiu/environments/aroma/modules/setup on your SD Card.
That one still works great for most of the titles, but from what I remember it doesn't have the latest updates for some of the games (unless they've fixed and updated that recently?). I suggest using either
NUSSpli (recommended) or
WiiUDownloader nowadays.