I'm working on a packaging box and I have a ready template.
I'm not an expert in Illustrator and I can't seem to fill the other boxes with colour.
If anybody knows how I can successfully fill color in, please let me know.
You're doing it right by using rectangles instead of using the dieline to fill the shapes.
Don't use the shape of the dieline and fill it. Add rectangles and your fills separately from the dieline.
It's better to keep the dieline ON TOP, on another layer of your filled colors because:
This is how it should look like when you're done. I used some other to make it more visible. The corners sharing a bleed should be at 45 degree.
Don't put white areas between bleed areas; fill everything with the same color to avoid "stamping" on press (for example, the bottom part is all filled with the same color, the bleed isn't limited to the edges of the dieline)
And where you know there will be glue, you should leave 0.375" to 0.625" of white area there; the glue will stick better this way!
I also suggest you read about rich blacks:
Either they are grouped and need to be ungrouped, or they are merged with Pathfinder and need to "expand" them, or they are compound paths that have to be released. If they are just stroked lines, you may need to draw a new box and fill it in their place. Tricky, but those are some solutions.