Wet the paper to expand the fibers. Then while it is in this expanded state, staple or tape it to anchor the edges in place. When it starts to dry, the paper will pull on these anchors as it shrinks and it will get “stretched” flat.
You can employ several ways to wet the paper: soaking, running it under tap, or using a spritzer.
Soaking is immersing the paper completely under water in a container. You do not need to soak it long. Even just a minute or two would be enough to get the fibers of the paper thoroughly wet. I would advise against soaking too long. I soaked a paper for 30 minutes once to see if there would be a difference. It resulted in some of the paper’s sizing coagulating in spots leaving areas that would not take in any paint.
If you do not have a container for soaking, you can just put the paper under tap or running water. Make sure all surfaces get wet (front and back). Try not to let the water jet on one area only as this may also damage the sizing in those areas. For bigger paper sizes, use the bathroom shower but take care to keep the paper from touching the floor and walls.
Spritzing the paper on the board also will work but takes a bit longer to wet the whole paper area well enough to start it expanding.