I use modroc as a simple base to work on.

I put it on a wooden base such as mdf, when it's wet I smooth it out, into the wet plaster I would either embed thread in a random pattern or lightly score out a pattern. To make things interesting coloured plaster can be dropped onto the still wet modroc base and trowelled in to make a smooth surface, no more than two colours as the white of the plaster will make three colours in total.

If I get really adventurous I would print a reverse copy of a simple painting onto ordinary A4.

In simple I mean something with a strong un-complicated pattern. When the plaster is wet and smoothed I place the print face down unto it and carefully smooth it out, the image is transferred into the plaster. I lightly score around the outlines following the image on the plaster.

After either method I let the plaster dry:Remove the pieces of thread and a random out line like a marble design if done right is there. The image if used is there the outline can now be filled in with coloured plaster, I use herculite hardcoat, I mix the dry pigment first and then add water. I fill the lines with the coloured plaster which highlights the design.

Image transfer:
It can be left like this or as I do the coloured body of the plaster can be strengthened. I use frisket to mask of the areas I do not want touched. The area to be covered is dampened, I mix up the coloured plaster and float it onto the areas I want it.

I would make up a mix of hardcoat and marble dust 60-40 this allows the plaster to be polished. I know the plaster alone can also be polished, but I get better results using the mix.

The procedure is repeated until all the areas are finished and polished, care has to be taken when masking an a polished are so the plaster is not ripped up while removing the frisket. I use wax to protect and increase the shine.

Cheaters fresco
Another method I have used with good success is transferring an image onto lime putty and marble dust plaster, trowelled over a mdf base. I have printed some images in reverse and carefully laid down the page on the wet plaster, gently smoothed out the page, the image transfers almost immediately.

I have three small works in the shed exposed to direct sunlight etc and the image has not faded.
Ilia's image in his above posts is perfect to try for the fresco method, hope I don't cause offence recommending this.

I hope this helps some one to try fresco and scag, I use the above as an introduction, to encourage me to learn more and try out ideas, they are simple techniques based on the principle of some of the real methods.