How to assign resources to part of a tasks

Hi,

Does anyone know how to assign a resource for a period in the middle of a task?

I have a number of tasks that are say 10 weeks long, however I would like to assign different resources to the first 4 weeks the middle 2 weeks and the last 6 weeks but cant work out how this can be achieved. I assign the resources as non modelled as I do not want them effecting the duration of the task.

Any help would be appreciated.