The first thing you have to do is to define the specific matches and their price for each one of your clients (People > Clients > Services > 'Update matches' button on the right).
Once you've clicked on the 'Update matches' button, you can select or add (by clicking on the +) matches and define their price.
This is what it looks like when you added matches:
Then you can add matches to a job. For that, click on Edit > Matches when you're in the job.
You can now start entering the matches applicable to this job and their number of words. The system will calculate in real time an income estimate (on the left) using the prices you defined at the client level for each match.
And you're done! Please note that the matches will appear on the invoice.