I would start by learning the AbstractArray interface. Also make sure you’re up on Parametric types since you’ll probably want to encode array size as a parameter. Then try writing a simple custom type from scratch. You can find many examples of simple AbstractArray types at JuliaArrays.
StaticArrays does what you want and would be a good example, but if you haven’t already written a custom AbstractArray type it might be challenging learning: StaticArrays is very sophisticated and has to jump through hoops to handle immutability.