You will probably want to create XML for FAST and have the Custom Deployer (or Event System) send the content to FAST.
First create the FAST XML that works and write a sample app so you can insert it into the FAST index from either a .NET or Java application. This does not yet involve Tridion.
Then write your Custom Deployer or Event System and pass the XML to FAST.
IF you are using a Custom Deployer approach I would suggest to contact Tridion Professional Services if you have not done it yourself or are not a Java programmer. The new Tridion 2011 Storage API provides new opportunities for the Custom Deployer. In the meantime I would suggest to append the FAST XML to the normal Page Content at the end, surrounded by some markers, and have your custom deployer pull it out of the Page output, send to FAST, then remove from the output before continuing.
This is a fairly difficult challenge for those who do not have serious Content Delivery / Deployer / Java skills. However, if you want to go for it yourself I would suggest taking at least 2 weeks of time to research existing solutions and experiment with the API.
Using the Event System might be a little easier - but your success or failure message will not appear in the Publish Queue and if the search index fails to update you can only log the failure and not pass the info back to users.