Help with edges

i have what i think is called a tapestry loom in the wiki and i am just starting to use it.

in the photo you can perhaps see that when i am using multiple weft colours (esp when picking everything third warp) i have messy edges - just from the different combinations of warp and weft the threads don't wrap neatly.

my teacher has tried to show me how to intertwine the two wefts so one holds the other in place, but that doesn't always seem to work.

thinking some, it seems like I could add an extra warp on each side and always loop round that. does that make sense? do people do that? is there a better way to handle this problem?

also, if there's any relevant vocab that would help me search for answers i would appreciate knowing that (i am learning in spanish, which has different names for everything).

ps i know my tension isn't great!