This is the short version.
You need the artwork in a suitable format for your CAD software as this is where you would create the extrusions of the shapes that make up the design.
A vector editing package like Adobe Illustrator or Affinity Designer (there are loads of others) will let you create the artwork if it doesn’t exist or edit pre-existing artwork to get it ready.
The main idea is to get the artwork broken up into like colours. If the logo is red, green, and blue on a black background you need to separate the red into one SVG, the blue into a different SVG, and the green - you guessed it, into another SVG.
With those three SVGs, you would import them into your CAD software. Fusion 360, OpenSCAD, whatever you prefer.
You then extrude the shapes to the depth you require. Add a base circle (presuming your coaster is a cylinder) and extrude that. Move everything where it should be once printed as the positioning matters.
In my experience, the following values make for a good coaster.
- Round base 100mm diameter
- Square base 100mm wide and deep
- Base thickness of 2mm (doesn’t sound like a lot, but it is)
- Your artwork shapes should be 1mm thick
- Print from the bottom up
- 0.2mm layer height
Export all of those four objects as individual STLs.
Import all four STLs as one selection into BS, when prompted, say ‘yes’ when asked if you want them as one assembly.
If you wish to print the coaster upside down to take advantage of the textured or smooth plate surface - in BS, select the assembly, right-click, and mirror on the X-axis. Repeat for the Z-axis. (Ignore all of this step if you want the top surface printed up.
If you are printing the coasters upside down, change the top layers to 3 and the bottom layers to 5, this will make the eventual top surface have the best quality colours, particularly if you used white.
Select the Assembly from the Objects panel and expand the parts. Make the assembly the same colour as the base, then change all the colour parts to their respective parts. Tip; I always suffix the filename of the exported STLs (and vectors) with the colour name, it makes it easier when you get here and you have many to process.
Order the parts so the most important part is at the bottom and in decreasing priority moving upwards. When you preview the sliced results, your coaster should look great (if you printed it base up, look from underneath the plate).
ALWAYS preview each layer of the sliced design as you may have confused the order of the parts and colours you thought would print, simply do not appear.
If you look through my library, you will come across some that I have printed one colour below another colour to produce a tint or a shade (lighter or darker). This is achieved by creating a single 0.2mm layer shape and placing it below the top surface by one or two layers. It depends on the colour above and how translucent it is. Testing is a must.
Some of my coasters have design objects that are not 1mm deep and some that require a single colour to be broken up into more than one part so you can order the layers better.
Download some of my models to see what I mean.