See also Mendo Base Parts and Mendo Measurements

Now you have three parts that you have designed. To put them together, you create a new file that you will save as BaseAssembly.skp. Bring in the first part you made, BaseWithGrooves.skp. Place the part at the origin of the three axes. It should snap into place at the origin. While the part is selected, you can go to the Edit menu and Lock the component. This will keep it from moving around later.

Next, you bring in the Upright.skp. Go to the File menu and choose Import. Then find the part. If you also place this part at the origin, you can move it into place with the move tool. You can also eyeball its placement, but let's go for accuracy. Use the Orbit tool to move the view around so you are looking at the origin. You can roll the scroll wheel on the mouse to zoom in and out, or you can use the zoom tool. Click on the upright component with the Move tool, it should have a blue box around it. Select the bottom corner closest to the origin of the axes. Move it up on the Blue axis a bit and type in 3/4. This should place it on the top of the BaseWithGrooves part.

To place the Mirror.skp part, do the same thing. Import the part and place it at the origin. Move it up 3/4" and then move it along the RED? axis 3/4" It should be now placed on top of the BaseWithGrooves and alongside the Mirror part. Save the file.