Once you have your ShapeOKO built, it's time to tune it for optimal performance.
Belt tension is a balance. The belts should be as loose as possible while still eliminating backlash effectively. If the belts are too tight they put unneeded stress on the machine. To tighten the belts, pull them just taught and tighten the clamps. Rotate the pulley one step, and see if the carriage moves. If it does, rotate it one step in the other direction. If the carriage moves again, then the belts are tight enough. If not, then tighen them a bit more and try again.
Motor current is another careful adjustment. The Syntheos wiki has a great article on setting the current.
In a nutshell, set the current as low as possible so that the motors can still move the machine without stalling. If the current is set too high, the motor drivers and/or the motors can heat up. The motors getting warm is normal. When the drivers overheat, they temporarily shut down until they cool down again. If the motors on your ShapeOKO shut down randomly, the drivers may be overheating, and so the current should be turned down.