I made a panel with 55mm long (wide) pieces and glued them together (I admit I did a sloppy job, not jointing them correctly and clamping it badly) and it now warps as a MF. The wood was also not on the dry side... OK, I made almost every mistake possible :)
1200mm = 48"
800mm = 31.5"
55 mm = 2.1"
Start with the wood you find lying around. Chances are it will be either cupped, curved, twisted or bowed. Ya can't use it that way. You must use wood that has been seasoned for 1 year per inch thickness in an environment that is below 12% relative humidity, normally that would be inside.
After the wood is acclimated and dry, checked with a moisture meter, you can begin to straighten and flatten your boards, checking them for splits and cracks and loose knots first. Mark out the lengths with chalk on those boards and remove the "bad" portions. Now you can begin. The boards must have one flat surface and one flat edge to safely lay on the table saw and against the fence. The only/best way to flatten a board is with a jointer that is wide enough to surface the entire width in one pass. You can use a router with a sled or a thickness planer with a sled, that why I said "best" way.
After you have one flat surface, you can straighten one edge using the jointer OR a tablesaw with a sled, OR a router with a straight edge guide. Use what ever you have that works best for you. Long boards are difficult to straighten on a jointer with short tables. Most 6" jointers are less than 60" long overall. You can joint a 60" board on that, but you will need some practice.
Once you have one flat surface and one straight edge, it's safe to use the table saw to make the opposing edge parallel and straight, but you will need some skill to get it just right.
After the edges are parallel, you can use a thickness planer to make both surfaes flat and parallel. The jointer has flattened the bottom, now the thickness planer with make the board a uniform thickness by removing material off the the top.
Now all the boards are flat, with straight edges and of uniform thickness. You can sort them for grain direction, and match and mate them side by side for the best look. mark one side with chalk so when you clamp them, the best side shows. Lay down your bottom clamps facing up, typically 3 or more. Coat the edges of the boards with a uniform amount of glue, not too much or too little, you will know with some practice. A roller applicator with help spread out the glue evenly. Then mate the boards together on top of your clamps. When you have them all in position, put the upper clamps on and just "snug" the center one first, making sure the ends are even.You may need to use clamps to hold the ends even as you clamp out from the center. You should cover your black iron pipes with painters tape before starting. This will make it easier to remove any glue drops AND will prevent the pipes from leaving black stains on the wood.
Do not use excessive clamping pressure, first snug then just hand tight. Wipe off the excess glue with a wet rag immediately OR wait until the glue just sets up and scrape it off with a sharp blade. Scraping will not leave a glue stain on the boards, but wiping may if you don't get them really cleaned off. You may have to hand plane the top if there are obvious offsets in the surface. That is a whole 'nother subject for another reply.......
That should do it for you, at least that's the way I do it as in this thread:
You tube videos showing the process: