Do you have corner or frame making clamps and/or clamps big enough to hold the sides together? You can see them in the photo of my 50g refugium build. You may notice the small white blocks in the bottom corners of my build. If you do the inside euro brace at the bottom first, you won't need the block like I used to help line things up. Also, do a dry fit before you try the real assembly. Use good adhesive silicone (I used black RTV silicone) and don't skimp when laying the silicone bead down. You can spend days scraping excess silicone out of the tank after it is cured, but you can't add more silicone into the joins after it's assembled and you find bubbles or air pockets. And don't squeeze all the silicone out of the joints when you are clamping it together.
Is that enough things to think about? And uncleof6 is dead right, build a small practice tank and then build a sump before you try to build the DT. Practice, practice, practice!
Good luck.
