RFTools Dimension Builder
The Dimension Builder is the device that you need to actually create dimensions. It can be crafted as follows:
Depending on the complexity of the dimension it will use some power and need some time to actually construct the dimension. If there is not enough power present the building will be delayed. You can also take out your dimension tab if you want to temporarily stop building the dimension (if you need power for something else for example). The dimension tab will remember the current progress.
As soon as the dimension is ready the tooltip on the dimension tab will change indicating the status and current power. You will also notice that the energy level of the dimension builder is suddenly emptied. That's because all current energy in the dimension builder is immediately given to the dimension. The dimension itself has a buffer of 40000000 RF. Don't attempt to go to your dimension if there is not yet enough energy in it. Wait until sufficient energy has been put in it (you can see the current level in the tooltip of the dimension tab).
Hint: if you shift-right click with the dimension tab in your hand you can see what random features were selected for creating this dimension.
Note on powering very expensive dimensions: the dimension builder accepts maximum 50000 RF/tick on every side. So at most it can get 300000 RF/tick if you input energy from all six sides and if you have conduits or energy cells that can handle that amount of RF/tick (a tesseract would work). Sometimes even 300000 RF/tick is not enough to power your dimension. In that case you can use multiple dimension builders and multiple (identical) dimension tabs. That does mean that you will need multiples of all the dimlets that you used to create that dimension tab as well.
Hint: the Special Efficiency Dimlet helps with reducing power usage.