This is so hard 
I've never done anything like this before, but I expected making a banner to be easier. All I've done is add a single character and I'm stumped... I have no idea what I want to put on it.
I have a few hints for you.
1-Think of a character you really like.
2-Look for proper images, holiday or winter themed.
3-Look for a nice and fitting background, like starry sky or falling snowflakes, make sure all images you gather are very high quality.
4-Imagine what other elements would look great with the composition, like starshines, gift boxes etc. and gather them.
5-Open Photoshop (if you don't have, just dl it, doesn't take much time to make it work)
6-Create a new image of 120x900 (transparrent and empty)
7-Upsize it 3 times (360x2700)
8-Start placing selected images in your newly formed workspace
9-Use pen tool to cut out what is not needed leaving you with only the image areas that you want to have (usually I use this to properly remove backgrounds or extract single characters out of images)
10-Use the miracle of Move Tool to properly re-size images, change their angle, stuff like that.
11-Place items in places you deem them fit.
12a-Save as PSD
12b-Save as PNG
13-close both files upon completion.
14-Open the PNG one
15-Downsize it to 120x900 and save
16-Upload
17-Post
18-wait for unfair or good feedback
19-profit from praise or hide from shame
20-most importantly you have to use imagination
