I am using tongue and groove planks for a desk. What’s the best substrate to use

Are the tongues necessary? Can I just take my circular saw right between the boards, cut them way, and then screw them back down afterward?

You've been misinformed. The point of tongue-in-groove planking is to keep the floor boards from twisting, slipping and sliding against each other and squeaking (or squeaking more in your case). You still need to secure a tongue-in-groove floor to the subfloor or joists.

Joists almost never squeak on their own. Squeaking comes from two pieces of wood rubbing against each other - usually because a floor hasn't been adequately secured.

Now if you DO need to cut away the flooring, then you should cut along the seams, and then buy Hardwood Floor Spline and rout out the old tongue so you end up with two grooves facing each other, and use the spline to rejoin the flooring when you reattach it.

But BEFORE you do that, I'd get a bunch of 2" coated screws and drop them through the subfloor into the joists below and see if that fixes the squeak. It's subfloor so you don't care about the face, and if it doesn't work, they're easily removed.

*Tip - use a hammerdrill when putting in the screws

First, a disclaimer: Make sure the wood you are going to use will stand up in the environment you're placing them in. You may be better off using these with the T&G on an indoor project. Weather outside will quickly weather non-pressure treated wood, and decks typically require thicker boards than you'd have inside since they don't have a sub-floor.

For long cuts along the length of a board, the best tool is a table saw. And the second best would be a circular saw with a jig. If you don't have plans for more projects like this, or simply need to save space, then the circular saw makes more sense. Setup a jig with a long straight edge that you can run your saw along and clamp it on top of your boards to be cut.

To smooth the edges, a router would be best. You could also use a block plane or sander. I'd be tempted to get a belt sander to make this project go quick and to give me the chance to cleanup the finish on the boards at the same time. A small orbital sander would also work and have other uses around the house, but it would take a lot longer and wouldn't be as useful if you want to cleanup the finish on the boards.

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