I think you first need to ascertain what paper is on the walls before you commence with your painting.
Some papers contain inks that can be reactivated by the application of water based coatings, causing them to bleed through your paint system, vinyls again are very problematic and emulsion paints should not be used as a first coat application.
If your paper falls into either of the above you will need to first seal the surface with either an oil based primer sealer, or undercoat, before your emulsion.
I appreciate the need to save money but painting over paper is not always straight forward.
Aside from the above in TheDecs post. There is the risk of the paper lifting. To mitigate this oil based paint will help to a degree. Zinsser BIN would be even better but is more costly. It is tintable and two coats might be enough to finish the job
You may have other problems such as joins being pronounced. If so you can cut through them and fill.
You might be better off in the long run if you strip the walls. Lining paper from real decorating stores is pretty cheap and "easy" to hang. You will then save money on paints (two coats of emulsion being sufficient) which will offset part of the cost of the glue, paper and filler.
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