Okay so I wanna make a poster that at least looks like I didn't cut it in half, glue it back together and leave it in a sauna for a while so I was wondering if there was an easy way to attach papers in a way that is barely visible and to print on the longer paper or just how to align the images better (if I print the papers separately and then attach) (Sorry if this is confusing)
There are two means of attaching two or more pieces of paper together neatly and precisely using either glue or tape. Here is a description of both.
Either way, the pieces must match along the edges that will be joined. The closer they match will determine how good the final product will appear. Lines should not jump and the colours must appear continuous without a break. There must be some overlap of the image because you will be working on only the parts of the images that overlap.
Technique 1. using tape
Using a sharp blade, cut the edges and tape the joint between them on the back of the prints.
A more refined variation would be to cut using a smooth wavy edge because the eye has more difficulty finding a break (or cut) if it isn't a straight line.
Another variation would be to tape the face of the joint with 3M Magic™ tape which has a matte surface and is less obvious than glossy cellophane transparent tape. This is more obvious than taping the back of the pieces as the tape is visible.
Technique 2. using paper cement (a kind of glue)
Using a sharp blade, cut the edge of one print and glue it to the face (on top) of the second print.
A refined variation of this would be to tear the edge of the top print instead of cutting it so as to make the edge as thin as possible where it joins the bottom print. This is called "feathering" the edge of the top piece. When done well, a feathered mosaic is difficult to tell from a single print at normal viewing distance for the assemblage. Also, when tearing, place the print face down and tear the back from the face of the print.
Tips for gluing:
• Use paper cement instead of glue since it contains no water which stretches paper causing the sauna-like distortion and wrinkling you dislike.
• Avoid "One Coat" type paper cement.
• Let the cement application dry completely before attaching pieces.
• Use pieces of "wax paper" between the glued layers to help you position them. You can see through it to align the sheets. When they are aligned, carefully slide the wax paper free allowing the glued layers to bond when they touch.
• Get a rubber cement "pick-up" square to remove the residual cement from the print.
Practice will improve your technique and results.
I think Stan has it here, but I'd also add: Print crop marks on the borders of your art; It's what the pros use to get perfect alignment. You can either trim them off when you are done, or print them in a faint (light blue or pale yellow) color that won't be seen from any distance.