I need to know how to create a cabinet like the attached sketch
None of the standard cabinets have the back corner cut off. You may be able to find what ypu want in the Sketchup Warehouse but I would do it (as in the example image) by creating myown top using the Floor tool.
1. Place the 3 cabinets in the model and turn off the Counter top on the Details tab.
2. Select any floor (eg Concrete), right click and select "by Picking Points" and draw the shape you want (if you have OBJSNAP in the Status bar ON, the floor will snap to the corners of the cabinets, making drawing the shape easy). If you want the top to stick out on any side, just select the centre marker of that edge and drag it out.
3. Right click the newly drawn 'Floor', select Elevate and enter the height of your cabinet top.
4. Right click again, select Properties, choose the Appearance you want (eg Countertop-Formica) and set the tickness you want on the Basic tab.
If you need a continuous back on the 3 cabinets you could use a short thin Wall or use a resized Custom Member from Interior Accessories / Decorative.
Hope you can follow that and that it helps.
Another alternative, if you have the tools & skills;
Export the cabinet design from TFP that is as close as possible to your desired result,
then go to another 3D program (modeler) and add/remove parts/lines/surfaces to get what you want. Ensure you name and colorize domains, and export....
import back to TFP and apply desired textures. This all can be done easily/fast but does require more than TFP.
thanks for your help