because the boards are almost certainly going to be cupped you cannot get them flat with a plane without loosing 3mm or so from the thickness
you start at the highest point removing material till you reach the join
small deviations will pull in during assembly
best to use a belt sander and rather than try and "flatten "the wood you better to try and feather the edges to match at roughly 1mm per 50mm
so if you have a 3mm step you feather it in over 150mm on the high side and a bit on the low side as well but never stop moving with a belt sander
doweling 1.5m L dowel every 200-250mm[6 dowels] gluing and clamping 4 sash 4 cross batten pairs glued 5th sash clamp used if required to pull together any loose gaps between boards wiped clean 20 to 25 mins
you start at the highest point removing material till you reach the join
small deviations will pull in during assembly
best to use a belt sander and rather than try and "flatten "the wood you better to try and feather the edges to match at roughly 1mm per 50mm
so if you have a 3mm step you feather it in over 150mm on the high side and a bit on the low side as well but never stop moving with a belt sander
doweling 1.5m L dowel every 200-250mm[6 dowels] gluing and clamping 4 sash 4 cross batten pairs glued 5th sash clamp used if required to pull together any loose gaps between boards wiped clean 20 to 25 mins