You have obviously made some mistake in promising your client a guaranteed return.
Anyhow now you note the following very carefully to save youself from any legal hassles.
1. Ensure that you have obtained proper skill for trading by experience and /or some theoretical knowledge /tranning . Keep some documentary proof in this regard.
2. You also must ensure and prove to the satisfaction of your client and (ofcourse to the court, in case of any litigation) that you have acted with reasonable diligence and used your skills in the entire trading activities on behalf of your client. You must have some analysis of share market and some home work done before any trading and you took moderate risk as per market practice. Keep such records for evidence purpose.
3. With all this communicate your client that you have taken all reasonable care before investing his money in any particular share or stocks. But markets turned negatively to your analysis and expectations . Submit a proper account to your client.
4. Inform this by registered post as soon as you can.
In case you need more clarification advice contact here for further advice.