Staking in the RepMe wallet will begin automatically provided you have coins to stake and the wallet is fully synced with the blockchain.
If you are having trouble staking, double-check that the chain is fully synced and the wallet is unlocked if you have encrypted it.
Please allow up to 30 minutes for staking to start. While this is not always the case, it can sometimes take this long. If you have waited more than an hour, and staking has not begun, try restarting your wallet.