There are several things to consider here; firstly the timesheets will need to be at the smallest increment, so in this example they will need to be Weekly timesheets in order to be able to pay the candidate on weekly basis. The Client Entry should be set to a Billing Cycle of Monthly and this will assist you when you come to produce the invoices.
Here is an example of how the assignment should be set. On the Main Details page of the Assignment Entry you should ensure the correct timesheet template has been attached:
On the Costing page of the Assignment Entry, set the Pay and Bill sections in-line with the currencies and rates:
The timesheet template of weekly will ensure you are able to pay the contractor on that basis and you simply bill the client at the end of each month, therefore all validated timesheets will wait in that area for you to bill when appropriate.