Before I explain how enriched uranium is made, let us see what enriched uranium is. Uranium that is found in nature consists of many isotopes of which approximately 99.3% is U-238 and 0.7% is U-235.
When uranium is used as a fuel for fission reactions only the atoms of U-235 undergo fission and release energy. The U-238 does not possess this ability. Energy is produced in a nuclear reactor when a chain reaction is started. This happens when the products of one fission reaction can lead to one or more fission reactions. As U-235 has a very small percentage, a chain reaction cannot be started unless its percentage is increased. This involves the process called uranium enrichment.
There are several ways in which uranium enrichment is achieved. The technique used initially was called gaseous diffusion and was based on the property that compounds containing U-235 and U-238 pass through semi-permeable membranes at a slightly different rate. The technique is extremely slow and inefficient and is being replaced by a method that uses a gas centrifuge.
When molecules of U-235 and U-238 are spun in a centrifuge, due to their higher mass U-238 molecules move to the outer portion of the centrifuge while U-235 remains closer to the center. The U-235 is collected from here. The process has to be repeated several times before the required increase in the U-235 concentration can be achieved, but it is around 60 times more efficient than the gaseous diffusion method.
In addition, many new techniques using lasers, vibrations, heat, etc., are being devised to be able to enrich uranium faster and more efficiently.