Flattening/storage 101:
Once you get a print, un-tube it, carefully unroll it and weight the corners so you can inspect it. DO THIS EVERY TIME YOU GET A TUBE -- don't just leave it in the tube and assume it's mint.
After inspecting, carefully and loosely roll it back up (it'll usually be very easy to let it slowly go back into a roll). Let your prints "relax" in this loose roll state for a while so that they're easier to work with -- how long depends on the paper, ink type, how long it had been rolled in a tube, etc. (1)
After they have relaxed, put a piece of foam core that's larger than the print on all sides on top of a flat surface. Unroll the print on top of the foam core. You can leave the Kraft paper underneath the print if this is only short term flattening, but if it's for long term flat storage, remove the Kraft and only use glassine or tissue in the sandwich.
Put glassine or tissue over the top of the print, then put print weights on top of the tissue/glassine to weigh down each corner of the print. (2)
Place the weights so that they are holding the print down sufficiently but so that the edges of them can be grabbed and removed via sliding from outside the foam core. (3)
Rest a second piece of foam core on top of the weights so that there is a gap in the middle to view the print. (4)
Position yourself so that you are eye level with the gap between the boards and can see the edges of the print where the weights are. (5) & (6)
Pick one weight to start with and slowly pull that weight out from in between the foam core pieces. As You pull the weight out, the top piece of foam core that was resting on the weight will start to lower onto the print. While pulling, you want to make sure that the print isn't moving, the tissue/glassine isn't moving, and that no part of the print is curling or folding under the top piece of foam core -- we don't want to cause creases or damage! (7) & (8)
After the weight has been fully pulled out, keep yourself at eye level with the sandwich and make sure that no part of the print is trapped between the foam core in such a way that would cause damage. The print and its edges should be flat between the two pieces. There should be a small gap still visible since the other weights are holding the remaining 3 corners aloft. If the print looks safely flat, place the weight you just removed on top of the second foam core board in that corner. (9)
Repeat this process with each corner's weight, doing both weights on one dimension before moving to the other dimension (don't go diagonally in other words). (10) & (11)
Once all weights have been removed, feel free to place more weight (such as larger print weights, heavy books, portfolios, etc) on top of the sandwich to sit there for a period of time, close the sandwich with binder clips, or use the top surface of the sandwich as the "bottom" layer of a new flattening layer. (12)