First off you need to obtain a Wii game ISO image, you will download it via Bittorrent, or Usenet, or use a premium service like USENext, or create it yourself. Once you have the game.iso file then you will need to get ImgBurn, its a AWSOMe FREE program. 

Download and install ImgBurn. You can Get this free program from their website: www.ImgBurn.com

Now you have the program, run it, and select this:

then you want to

Then Browse over to where you game ISO file is, highlight it and click open 

Then you want to set your burning speed to 2x for MAXIUM compatibility.  

 Make sure you blank DVD-R is in the DVD burner, and them click the arrow

Once burned you can play the game in your modded wii!

Have fun!