Sounds like a great idea. You could mark it off into square feet with twine tied to some small stakes or whatever works for you. But I do find it most helpful to have my squares marked so I can follow the guidelines for spacing.
Best of luck to you.

There is no problem with doing that. You could (if you want) augment with some peat, vermiculite and compost if you wish, but intensive gardening using the SFG concepts is fine in any garden, raised or otherwise. Oh, and I do mostly visual spacing now anyway.

Here is a link that might be useful: Sinfonian's SFG

If you look at the cover of Mel's original book, that is exactly what he did. Taken a big garden and put boards down to separate into 4x4 beds.

I would turn in that compost instead of putting it on top. It can improve the whole depth of your soil as it nurishes your plants.
When it's time to plant, the simplest thing to do is to just draw out the squares on the ground with the edge of your trowel.

As a newbie to SFG I found that having my squares clearly marked was very helpful. For you folks who've been doing it for a while you may not need that. Seems to me that one of the most important parts of it is spacing. You don't want to waste when you might have been able to plant more, and don't want to squeeze those crops in so tight they don't do well. Maybe I'm just a little obsessive/compulsive but my squares are clearly marked.