Select the custom shape tool, set the layer option in the top left of the control bar and then goto the 3D menu and select new extrusion from layer. Modify it. Select either of the options, layer or path
That is the start point. You have your model but now you can apply all kinds of presets to the artwork and modify it in all kinds of ways. I must admit, I wish it was a full blown tool to manipulate every aspect of the model but you can always export it at this point and manipulate it in applications such as Blender etc
The key panels are the properties and the panel and also the various controls to modify the rotation, scale etc. Goto the panel. Select the entry (it may have a crazy name in the panel - there appears to be no rename feature). You can see they have very long names with front inflation and front bevel etc
You will notice that it breaks down to a number of different materials such as front inflation, front bevel etc (it is a pity there appears to be no just overall material preset for the lot so you have to select each item in turn and change them, however select the main item and go to the textures entry in the layer, you can change the overall texture there via the default texture) such as by adding a gradient to the textures base color.
Goto the properties panel with the layer selected and change the depth. I am never sure why they have never added a preference for this as the value is always massive and far greater than I ever want such as perhaps 50px instead of the current default. I always have to re-size the depth.
There are a number of presets, they become available depending on which item you select in the panel or the layers and the presets are then available via the properties. The presets are useful but you can also just set the values yourself and some cases, just as easy. The range of materials available is also fairly limited which is a pity especially when you compare it to the range in the Adobe Dimension application. Select the entry in the layers panel and then select the front inflation material (all great names !) and then go to the properties and you will see a presets browser on the right side of the panel.
This is independent of the model, you can also change the current view and how you see the model and there are presets for that as well as FOV (field of view) and depth of field etc
Again, independent of the model but without you won't be able to see the model. Go to the panel and select the light and you can add more than one light and there are three types of light. You can, of course, over do the lights making it so dark you can't see anything or so bright you can't see anything as well, lighting is probably the key thing in any work. Doesn't come with many lighting presets though which is a pity.
One of my favorite and slightly annoying features is the instances feature which you can find by going to the panel and selecting the model and right click and instance of object. An instance means it is a linked copy to the original and you can make changes to the original which then are changed in the instance. The instance feature only creates one object at a time so if you want 50 instances of your object you have to right click and instance and then repeat that over and over, why not add an instance dialog ? and allow 50 instances. You can move, rotate, scale them independently of the original, extrusion etc is not independent so you can change them all at the same time.
You can deform the model via the selected entry in the panel and properties and the deform panel. It comes with a useful set of presets as well but you can also alter the extrusion (why is it there as well ?) as well as the twist and taper and bend and shear etc and many weird and wonderful design can be created. It is not as useful as being able to manipulate the extrusion directly though as a true three dimensional artwork but some unusual designs can be made.
One of my favorite features is the sketch renders, click on the scene in the panel and then to properties and select one of the presets. You can also got for the surface and lines and points and create many variants of renderings from this. Goto the surface entry and right side 'style' and select the bottom entry, sketch. You can change the texture, you can even use the current brush though to be honest most of the time the result is never that great. The sketch does take a little set up to get the best sketch result and it will never a proper sketch render or ease of setting up. Of course, once you have created this you can always return to just normal layers and layer menu and rasterize, which you can do at any time. You can use the generate from brush to add your sketch render, it is not quick and generally result always seems to be a solid color.
You can also create a lovely render of the model, this can be slow depending on your render setup. With the model selected, go to the 3D menu and render The design can then be used just as a normal layer and combined with other layers etc
The model can be exported for use in another application. With the model selected, go to the 3D menu and export layer and select OBJ format etc
It should be noted that more and more of these features are being removed from the application so you may find they no longer work. I would suggest now using the powerful Dimension tool if you want to use these features in the Creative Cloud