sophi50943 normally by activating the replacement SIM. If you have activated it check to see if your SIM will work in a different phone.
Also note that barring a SIM will also bar your phone. If you haven't removed the bar on your phone you should contact the agents for help by submitting this form Block or unblock phone
Agents reply within 4 hours in the majority of cases but can take up to 24 hours, in the worst case, to reply (but they do work 7 days a week from 8am to 11pm). You will see when an agent has replied in the "Messages: From Agents" box which is half way down the right hand side of your giffgaff dashboard or go here Ask an Agent messages