Here is what to do; When you cut down the trunk cut it up in 6' sections, I try to make my cuts at the nodes, not between them. I let the trunk dry whole, on some bamboo if you cut out the slats green, they won't dry flat and will cup up in a U shape and be worthless.
To dry the trunk quickly it is important to knock out the inner nodes so the trunk has airflow through it like a chimney. I use a hoe handle for this, the nodes knock out easily. Sometimes I run a bead of super glue around the ends of the cut to minimize cracking while the trunk dries.
When your bamboo turns tan it is ready to cut or split into slats, drying might take a month in the summer, I keep my cut bamboo out of the sun in my shop while it dries.
I don't split my bamboo into slats, I pop a chalk line on it for the first cut and run it through my bandsaw, I use a rip fence to cut the slats to width. The slats will still have left over node pieces on the inside, I cut these off with a bandsaw.
The bamboo is way too thick at this point, I stand it on edge and run it through my bandsaw slanted, using the back as guide I cut the thickness on the belly on one side to 1/8", flip the slat over and cut down the other side the same. Don't try to cut the belly thinner side to side in one pass
bad things will happen, the slat is too flexible at this point for a controlled full belly cut.
At this point I run the slat on my belt sander to get the peak down the middle of the belly flat and then make a few passes through my jointer to finish flattening the slat. I have left the jointer out and done all the flattening on my belt sander using a 36-grit belt, it goes pretty fast. I have a dust collection system and found the bamboo "wool" that comes off the belt sander will stop up the piping on my dust collector if I sand a lot of slats.
I always had a bandsaw, before I had the other power tools, I did the flattening and thinning with hand tools. I hadn't cut my own bamboo at that point and was working off commercial 2" wide, very thick slats that I bought.
Here is what I started with and ended up with;