Welcome to the Forum, Anneli! 

* The size of the pot, the amount of paper:

Just use what you've got. To soak the paper to soften it, use any kind of tub that can hold the amount of paper that you think you will need. After 24 hours or so or soaking, use the largest pot you've got and half-fill it with water.  Tear some of the paper you've soaked into squares and add them to the water.  Usually, you need quite a bit of water to break down the paper, so it should look like soup to start. If you think it can accept more paper, add it.  If the mix is too thick, you may have to take some out and put it into a bowl for the next batch. After you make a batch or two, you'll learn what is best.

If your pot isn't very large, you may need to make several batches, depending on the size of your project. 

If you overestimate the amount and have pulp left over, squeeze out as much water as you can and let it dry thoroughly, then save for another project. (Do not add adhesive.) Then, all you have to do is add water and adhesive to the dry pulp. Wet pulp will mold if left wet more than a few days, depending on air temperature (warmth makes mold grow faster).

Some PM artists make very large batches, enough for several projects, squeeze out the water, dry it thoroughly, and put it into paper bags until they need it. Then, when they get an idea, they can start right on it.

* Claylike pulp, dough, or mushier....

The more water that is in your pulp, the longer it will take to dry.  I would make it only wet enough to conform to my design.   Sometimes, I have laid down some old towels, put a piece of window screen over it, and pressed my lump of pulp flat with my hands or a rolling pin (as for pastry), then applied it to my 'skeleton' or underlying form. You can also make patties of an appropriate size, and gradually cover your form, pressing small lumps of pulp into any gaps.  If you have some kind of mesh strainer that is approximately the size/shape of what you are forming with pulp, you can press your pulp into/onto that and squeeze out much of the moisture.

Doing some initial experiments before an important project can save you a lot of time. Soon, you'll find what works for you and what doesn't. Some methods work well for some things, others work better for others.

One thing about pulp:  Some people try to form one solid lump of  pulp into a form, and it never seems to dry completely. Moisture trapped in PM is not good --- it will always ruin your hard work.  Instead of making it from a solid lump, put some kind of armature inside, even a crumpled lump of aluminum foil, a plastic egg, a plastic bottle or jar (capped), etc.  A large lump of crumpled aluminum foil can be beaten into proper shape with a hammer. :-)

* I read that it is possible to use molds and cast things. Plastermolds? Doesn´t the pulp slick to it. Or is it easy to remove when dried?

Plaster is a popular mold for two reasons:  the porous plaster will absorb some of the moisture from the PM, thus helping it to dry faster.  And when the PM dries, it shrinks, and normally pulls away from the mold as it dries, so it is usually easy to remove.

Anything that doesn't have undercuts that will make the PM cast get caught in the mold should work very well: plastic, glass, metal, etc. But with these, you will either have to cover the item with a mold release (petroleum jelly or others), or cover and tape plastic wrap/cling film to the item. When the PM is dry, most adhesives won't stick to the plastic wrap and it can be peeled off.  The EXCEPTION here is if your PM gets worked into creases and flaps in the plastic --- these can be more difficult to release.

If you are using the outside of a mold like a vase, let the PM dry thoroughly, then cut the PM off with a razor blade and then fasten back together with adhesive and more PM.  This method seems to work best with strips, rather than pulp.

You will want to consider buying Jackie's e-book "Papier Mache Art and Design", as it has a lot of information and photos in it. See the notice on the right side of the page.  Jackie owns this site.

Have fun and do some basic experiments.

Sue

I don't soak my paper anymore...i use white paper, like copy paper, and envelopes, that i re-cycle...i tear them into pieces ( not real small) and put them into a blender of  hot or very warm water. they chop up real nice..I used to soak the paper, but tried it this way one time and have never soaked my paper again..Newspaper is very hard on the blender compared to white typing paper...
best to use a old blender you don't mind blowing up. Sooner or later it will give up the chop. I look for them at yard sales and at Goodwill, and buy them  ahead so i always have anouther on hand. I found that if you don't  use a lot of paper. 6 sheets is plenty...at a time, the blender will last a long time..I then strain it through a piece of cloth or pillow case, so i can hand wring the water out...once i get all i can out of it. i add my home made  glue and use a mixer to mix it all up.it gets better if it sits a little while before you use it.
it would be hard to measure how much to use.., once you make it several times you will get the knack of knowing how much glue to add to such and such amount of paper..I think everyones feel for the pulp is a little different..