There are many techniques for producing good quality casts. They all involve three processes.
Preparing the specimen for casting
The preparation is the most important part of the process. This requires the making of a mould into which the casting medium (plaster, resin or similar) can be poured. The traditional process is as follows;
Plaster of Paris is a quick option. To use this casting medium, mix a solution of water and 10% PVA adhesive in a bowl. Add the Plaster of Paris until it is mixed to a creamy consistency and pour gently into the mould. The plaster should dry in an hour or so to a hard consistency, being reinforced by the PVA. For colouration, a powder paint could be mixed with the Plaster of Paris dry powder in advance.
Resin is also a good casting medium. Thoroughly mix the resin and 2% catalyst in a cup. Add colour to the mixture if required. Pour into the mould and allow to set. Most resins will set hard in an hour, but for some specimens it may be best to leave longer. For large specimens the resin may need to be reinforced with a fibreglass mat.
Some resin casting techniques use a gelcoat resin, which can be coloured, to coat the inside of the mould before adding the structural resin. Care needs to be taken to mix the colour of the gelcoat resin so that it can be repeated. If the edge of the cast needs grinding smooth, more coloured gelcoat must be added to mask the ground sides.
Additional colour can be added to the cast using enamels or powder paints. Varnish to seal the colours after they have dried.
Using ground stone is also a good option for some casts. Some types of plaster made from finely ground stone are best baked at high temperature in an oven to ensure that the particles fuse to form a hard structure.
Don't forget to clean the fossil and re-fix any labels afterwards.