There's nothing wrong with cheating a bit!  ;-)

Find a thrift shop vase or box in a shape and size that pleases you.  Wrap with plastic wrap/cling film and secure with tape.

Tear some paper strips, apply glue (brush on, or dip and remove excess between your fingers), then apply them to the covered vase.  Start with about three layers, overlapping each a bit, and running the strips in a different direction for each layer, smoothing each layer well.  Let dry thoroughly.  Then take an X-acto knife or box cutter and cut through all three layers, so you end up with two vertical half-vases (removed from the original vase).  Use pieces of masking tape to stick them back together.  Get some more strips and glue, and add another layer or two of paper strips so the whole piece feels strong and secure.

Now you can make a choice: 

1) Continue with strips until the vase feels thick enough and solid enough for your purposes, or
2) Make some liquified paper pulp in a blender (liquifier in Britain?), strain most of the water out of it, add some glue, and apply to the PM vase.  If there is still too much water in it, it could make your paper vase collapse.  If it doesn't have enough water, it will be difficult to apply and smooth.  Let dry for a day or so, then burnish with the back of a spoon to smooth it.

Use a piece of wood wrapped with sandpaper if further smoothing is needed.

Coat with latex or acrylic wall paint (emulsion) or regular acrylic paint, then decorate.

But somehow I feel that this isn't really what you meant.  Did you mean an interior mold, where you press the paper or pulp INTO a mold?  This is trickier and more difficult and more time-consuming with the more complicated shapes.

Boxes are easier.  Line a box of the proper size with plastic film or rub vegetable oil all inside a sacrificial cardboard box until it looks damp.  Don't soak it with oil, or the box will become limp and useless.  Press your pulp into the box.  Try to get the thickness fairly uniform, which is more difficult than you think.  Let dry well.  Once the PM inside is fairly firm, carefully cut or peel the box from your PM casting.  If the box isn't cardboard, use the plastic method and use the excess (hanging over the outside) plastic to pull your casting out of the box.  If it doesn't pull out easily, you will have to wait until the PM is completely dry.  When dry, it usually shrinks a bit so you can take it out.

If you want to use a plaster mold, you will probably have to make it, unless you already have a pre-made mold. And making a plaster mold isn't quite the easiest thing in the world.  Not the most difficult, mind you, but you would need to read up on how to do it. 

There are also flexible molds you can make, and they're more complicated than the plaster ones (also not impossible, and they tend to be more expensive, from somewhat to very).  PM castings are easier to remove from them, but you still have to be careful with your choices of designs, as they flex, but they don't stretch much.

If you just want to use the inside of an existing shape, you may run into the difficulty of not being able to get it out of the mold, if the neck is smaller than the body of the vase (etc).  An open bowl with an edge that goes straight up, or a bowl that opens outward would be fine. 

With some things, you might be able to cut the PM and remove it in pieces, but a narrow-necked vase isn't one of them unless it has a WIDE neck.

I've heard of Li-qui-che, but have never seen it, so I don't know how liquid it is.  If very liquid, you would need a plaster mold of the same kind you would use for making ceramics, which can be relatively expensive to buy.  Again, you could make one, but you would have to know what you're doing.

So, if you think about it for a bit, you can see why I suggested applying the PM to the outside of existing pieces.  The fact that you can cut it off fairly easily makes a lot of difference.

But if I am ENTIRELY misunderstanding what you had in mind, please say so, and if you can, post a photo or a link to a photo of the basic type of design you would like, and we can go from there.

And don't try to be perfect in the beginning.  As with anything else, papier mache takes a bit of practice.

And welcome to the forum!

Sue