Its almost impossible to create a banner in CustomizeMii that will brick your wii, as it has security checks to prevent you from doing just that, and if you are installing wads you should have priiloader or bootmii as boot2 installed anyway, so don't worry about bannerbricks. Anyway, here are the basic steps to create a channel.
in the "Source" tab choose the wad animation you want, you can see what they look like using the "preview" button.
in the "Title tab enter the title for the channel.
in the "Options" tab under "Title ID" put in a unique, 4 character title id.
Under "New DOL" put the dol for your emulator, if you want to make a forwarder channel, click forwarder and add the path to the dol on your sd card.
Under "New Sound" put the sound you want for the background music of the channel.
in the "Banner" tab you will see all the images used for the banner you selected. By previewing them, you should be able to see what part of the banner they are for, and then replace them with your own pictures.
in the "Icon" tab, do the same as in step 4, except this time you are editing the icon, not the banner.
Click "Create Wad" and then install it on your wii, enjoy!
If you have any questions, feel free to ask, the "Help" tab in CustomizeMii also has some useful info.
Hope this helps, and sorry if I made it look difficult, its actually quite easy