Select "Instance" mode from the toolbar. The editor options dialog will open that additionally prompts for some instance parameters. The most important one of course is the cell that shall be placed. Geometrically, the rotation angle can be specified, the mirror option can be set and the instance may be specified as a regular array. As an array, the instance represents multiple placements of the cell, arranged in regular grid which is specified by the two axis vectors and instance counts in each direction. Don't forget to click "Apply" to take over the current settings. If the dialog has been closed unintentionally, it can be reopened with the F3 shortcut.
To place the instance, move the mouse to the desired location and left-click to place it. While moving, the right mouse button can be used to rotate the instance by 90 degree counterclockwise. Press the ESC key to cancel the operation.
Starting with version 0.22, KLayout supports libraries. Libraries provide cells from the outside of the layout. These cells are imported into the layout and a copy is stored inside the layout. Still, KLayout maintains the reference to the original layout, so if the library changes, the cell will be replaced when the layout is loaded again. Cells imported from a library appear as "Library.Cell" in the cell tree and the layout. Here, "Library" is the library name and "Cell" is the cell name.
The library is selected from the pull-down box to the right of the cell name. You can use the "..." button to browse the cells available in the selected library.
Libraries are a convenient way to provide common layout building blocks. In addition, KLayout now also supports PCells (parametrized cells). Such cells do not have a static content, but instead they are created dynamically from a piece of code using a set of parameters specific for the PCell type. For example, a circle has two parameters: the layer where the circle should appear and the radius of the circle.
PCells are provided by libraries. If a PCell is selected from a library, the instance properties page also offers a panel to edit the PCell parameters. What parameters are available depends on the type of PCell.
KLayout comes with a standard library "Basic" which offers some basic curved shape types and a text generator.
KLayout offers a unique feature for the PCell implementation: a PCell can employ "guiding shapes". "Guiding shapes" are shapes that do not appear as layout themselves but are used by the PCell to derive its geometry from. For example the "rounded path" PCell of the "Basic" library uses a path as a guiding shape. This path is manipulated to obtain the final shape.
Guiding shapes are drawn on the cell box layer and can be manipulated with the normal shape manipulation functions (in particular move and partial edit). Also, the shape properties can be edited via the "Edit/Properties" function.
To learn more about libraries, read About Libraries. Read About PCells for details about PCells.