A single missed dose is common and usually not serious. The general rule is to take it as soon as you remember, unless it is nearly time for your next dose, in which case skip the missed one and continue as normal. Do not take two doses together to catch up.
Missing doses repeatedly can make the antibiotic less effective and give bacteria a better chance to survive, so try to take them at evenly spaced times. Setting reminders can help.
Specific advice varies by medicine, so check the patient information leaflet or ask a pharmacist if you are unsure. If you have missed several doses, contact your prescriber for guidance.