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There has been tremendous progress in the performance of optical frequencystandards since the first proposals to carry out precision spectroscopy ontrapped, single ions in the 1970s. The estimated fractional frequencyuncertainty of today's leading optical standards is currently in the $10^{-18}$range, approximately two orders of magnitude better than that of the bestcaesium primary frequency standards. This exceptional accuracy and stability isresulting in a growing number of research groups developing optical clocks.While good review papers covering the topic already exist, more practicalguidelines are needed as a complement. The purpose of this document istherefore to provide technical guidance for researchers starting in the fieldof optical clocks. The target audience includes national metrology institutes(NMIs) wanting to set up optical clocks (or subsystems thereof) and PhDstudents and postdocs entering the field. Another potential audience isacademic groups with experience in atomic physics and atom or ion trapping, butwith less experience of time and frequency metrology and optical clockrequirements. These guidelines have arisen from the scope of the EMPIR project"Optical clocks with $1 \times 10^{-18}$ uncertainty" (OC18). Therefore, theexamples are from European laboratories even though similar work is carried outall over the world. The goal of OC18 was to push the development of opticalclocks by improving each of the necessary subsystems: ultrastable lasers,neutral-atom and single-ion traps, and interrogation techniques. This documentshares the knowledge acquired by the OC18 project consortium and givespractical guidance on each of these aspects.

Guidelines for developing optical clocks with $10^{-18}$ fractional frequency uncertainty