This is a fine art that take lots of practice. Introducing moisture to paper and cardboard to loosen the fiber is needed. To much and you will ruin what your working on. Cardboard can take a lot more. First off only use distilled water. Also building a humid chamber with a meter is best, so cardboard could be in 20% humidity for 30 min but a manual only needs 10 min. Then you can use standard iron to really heat things up but no more then 200 degrees. This will flatten out the creases. Then you need to get the box or manual under a very heavy weight for at least 10 hours.
Now there are many other steps like special papers that you put between the iron and damaged paper. I dont use an iron much and have a digital heat press so i can control heat and pressure. Things to consider like staples, if you press them the entire staple will transfer its shape across all the pages. Also things need to be as clean before you start this process or as water and heat are introduced you could cause smears and other blemishes.
Like someone did above is just fine for very good results. I am really moving into the next stage of restoration and making these things best they can be.