any advice on big constructions will be welcome
First thing you need to know is that Flarechannel was a huge project, I forget how long it took him but I would most emphatically not recommend something of that scale as your first megaproject. That said, jumping in to unknown waters headfirst is the true dwarven way, so here's the advice I have to offer.
1. You can never have too many masons. Nearly every dwarf in your fort should have masonry enabled, possibly metalsmithing and carpentry as well if you want wood or metal constructions. Use workshop profiles to prevent the unwashed masses from wasting your workshop space.
2. Designate stockpiles near the construction of the material you want to use. In addition to the obvious benefit, this makes it easier to find the right material in the construction list thingy.
3. Plan out each step beforehand, preferably on graph paper. There's nothing worse than discovering that your pump tower doesn't have room for the mechanisms to be added, or that you misestimated an angle somewhere and need to rebuild the whole thing.
4. When building structures with traditional rectangular floorplans, start from the corners and places that can't be reached.
5. Deconstructing takes much longer than constructing.
6. Despite the above, the best way to make complicated constructions is to fill the entire space you want to use with stairs, then gradually replace the stairs with whatever you want to be there. Be warned that this takes forever.
7. If you have a powerful computer and a fortress that minimizes pathfinding, you might be able to keep your FPS above 25. Possibly the hardest part of megaprojects is surviving the FPS hit, don't give up when you realize that the new inverted magmafall drops your FPS to 5.
8. Your military must be well-armed and vigilant. Your dwarves will be spending lots of time outside, so your construction yards need to be well protected.
9. You may want to consider building temporary habitations, especially dining halls and bedchambers, near the construction yards to reduce travel times.
10. Be careful. Its easy to forget that constructions can't be supported by bridges, or that you can't construct stairs down over empty air, or that your dwarves need food to live.
11. If you want to make things out of metal, glass, soap, or something else unusual, start large scale production long before you start working. Its amazing how quickly a glass pyramid can drain your stockpiles if you haven't spent the last few decades preparing for it.
12. If you're doing anything involving water or lava/magma, make absolutely sure you understand the flow mechanisms perfectly. What you think you know from standard gameplay is very different from what you need to know to create a fluid-based megaproject.
Anyway, thats all I can think of, good luck!