Please, refer to the “Removing starting cube and camera in Blender” post to obtain a clean Blender startup.
Shift + s -> Cursor to Center
Lower horizontal command bar. Add -> Mesh -> Plane
Click on the “+” which is in the upper right corner of the screen. The “+” appears on the left corner with respect to the right vertical command bar. After having clicked on “+”, a new menu will appear.
At this point, the plate belongs to the xy plane of the red(x)-green(y)-blue(z) coordinate system appearing on the image. If you would like, for example, to rotate the plane around the y-axis of 90° so that the plate belongs to the yz plane, then just go to the newly appeared menu, go to Rotation -> Y and select 90 instead of 0. Now the plate belongs to the yz plane.
If you would like to change the size of the plate, then go to Dimensions of the newly appeared menu. Now, even if the plate belongs to the yz plane, the dimensions to change are X and Y, since they refer to the original own coordinate system of the plate.
If you would like to change the position of the plate, for example moving it to x = 3, then go to Location and then set 3 on the X box. The coordinate system of location refers to the red-green-blue coordinate system of the image.
To create the mesh, then switch from “object mode” to “edit mode”. This is possible by acting on the lower horizontal command bar or by clicking in any part of the image and then pressing “Tab” (“Tab” switches between object and edit modes). In the edit mode, then CTRL + t.
The plane will be divided into two triangles. After that, go to Mesh -> Edges -> Subdivide (Mesh is on the lower horizontal command bar). Repeat the subdivision the number of times you need, until the mesh is sufficiently refined.
Finally, File -> Export -> Stanford (.ply)
Remember that, if you want to turn your object, you have to simultaneously press the left and middle mouse buttons.