After making the investment you should ensure that you receive your share certificate like any other equity investment.
The company will however need to give you a document known as the S-EIS3 certificate that they receive from HMRC. You will need this document before you can claim your tax relief.
It is important to note that the company will not be able to provide your S-EIS3 certificate until they have their minimum trading requirements and submitted their Compliance Statement to HMRC.
Their minimum trading requirement before they can issue your S-EIS3 certificate for SEIS is the earlier of 4 months trading or spending 70% of the capital raised, and for EIS it is 4 months of trading.
If this is not received by 31 January following the tax year you made the investment you will need to file your return without claiming the relief. Once the S-EIS3 certificate is received, you can then claim the relief via an amended return.