Sure, here are the steps:-
1. Sign up/Login to Yahoo! pipes
2. Click on "create a pipe"
3. Open the sources drop down on the left (probably already open), then select a "fetch feed" block and drag one onto the workspace. Click the "plus" button in this block and add as many URL's as you need for the various google alert feeds you have. You can get these from your Google alerts account - each alert has it's own feed URL.
Add some pattern matching/negative filtering if required:-
4. Open the operators drop down on the left and select a "filter" block and drag it onto the workspace.
5. Add some pattern matching rules. For PaulPA's problem above you would create a couple of rules with a negative text match on "press release" in both content and title.
As with any negative pattern matching you might lose some wanted material in the cross-fire so you need to bear that in mind. My Press Release example might be better done using RegEX on the typical first few characters of a press release being a date time and location followed by a double-dash - haven't given it much thought as I actually require press releases. But you get the gist of it I'm sure.
6. Connect the blocks by clicking and holding the little dot in the feeds block and drag it to the filter block. Then click the filter block dot and drag that to the "pipe output" block that will have been auto-created for you.
7. Save it and give it a name
8. Double-click the "output" block and it should run in debug mode and give you output at the bottom of the screen. From here you can tweak your filter settings (or start playing around with some other blocks to help filter content).
9. When you're done go back to My Pipes main screen where you can get the URL for it. Click on that and you'll get a little RSS icon in the resulting page which you can use to put in your RSS feedreader.
This works really well for me. I have about 6 negative match keywords which have really helped get my alerts down in number to a manageable and far more useful level. Saves me trawling through them manually.
Quick heads-up - it seemed to take about 48 hours for me to get anything at all in the feed and then it ran fine. Not sure if that delay is at the Google end or the Yahoo! end. But results were not instantaneous....