1) create a flat frame outside the piece to be flattened. Could be a couple of 2x 's. The essential thing is that they should both be in the same plane both longitudinally and widthways.
2) Build a bridge to hold a router that will slide non the two side pieces
3) procure a 1 inch/35 mil bottom cutting bit for router.
4) use bit to cut across the top slightly overlapping the several cuts.
5) Finish with #7 or #5 set very finely as there should be little to remove...possibly use a scraper plane instead.
beech... no dis-respect intended but..... och away wi ye man... by the time ye've scuttered aboot buildin yon contraption I could ha'e the job done and the kettle on plannin the next one..... sheesh.....
McLuma...
if your budget is limited to just the 1 plane... get a jack... and make it a good one... it has its work cut out for it... either a 5 or a 5 1/2...
first point... if you rip the top and run it through the thicknesser...it'll come out every bit as warped as it went in.......only it'll be thinner....
you need to be able to read the panel... find out where the high spots are and mark them, 'cos its them that need to be planed down... To see what's going on you'll need a good long straight edge and a pair of winding sticks... use the winding sticks to see where the warp is and to judge just how bad it is...
If the degree of warp is worse than the minimum tolerable thickness of your material... stop there... it'll never come out right...
assuming the warp and twist is recoverable... start by getting your blade as sharp as possible, set the frog fairly fine, adjust the blade to take a reasonable shaving, clamp the board down (use wedges under any high spots to stop it rocking if you need to) and start to work down the high spots. Take shavings across the grain, preferably working in overlapping diagonals until you've roughed out the worst of the high spots... turning the board around to plane from the opposite side should produce a herring-bone effect when planing across the diagonals.
stop periodically (for a breather if nothing else), check your progress with the straight edge and winding sticks to gauge how far you've yet to go...
once you get to the point where you're reasonably happy that the top's free of cup, twist and bow, re-tune the plane; this time you want to adjust the frog for a real fine shaving. If you haven't done so already, clip the corners off the blade by drawing it back over the stone gradually rocking the blade outwards quite aggressively; this will rid the blade of its tendency to leave harsh lines at the edges of each shaving.. set the blade to take as fine a shaving as you can (any finer and it'll be producing dust) and gradually work the board by planing along the grain and working with the grain (the plane will tell you in no uncertain terms when its working against the grain)...
Again, check your progress periodically with the straight edge and winding sticks; pay attention to areas that case the straight edge or sticks to rock, see where the high spot is and smooth it till its flush...
Planing isn't rocket science; its a slow, patient, methodical process that will generate the desired result provided you take care of the high spots...
remember that five minutes spent checking is far better than an hour spent removing a low spot that wasn't already there when you started...
Last point... once the top's nice and true, you need to figure out why the hell it warped in the first place... from what I've read, this wood should be stable when its been dried to a moisture content of around 9%; your reclaimed stock should be pretty close to that figure by now so I'd guess that the cause was either incorrect finishing (the top has only been finished on one side for example) or working the wood before it had had sufficient time to acclimatize to the conditions in your surroundings...