Stake SEXP to mint sUSD. The other synths can be obtained by trading sUSD for them in the Synth Swap. Your C-ratio is a ratio of the value of your locked collateral and your share of debt. When minting, you need 4 times as much collateral value as the value of the sUSD that will be minted -- the C-ratio will be 400%. Make sure to maintain your C-ratio at or above 400% to claim fees. This is automated by keeper bots, so you don't have to claim manually. If your C-ratio drops to 200% or less, you will be marked for liquidation. You have 3 days to bring your ratio back to 400% to avoid liquidation. You can increase the C-ratio by adding more collateral or burning sUSD.
When your ratio is below 400%, you also have the option to self-liquidate some of your collateral. This will increase your ratio automatically by removing some of your debt at the cost of some collateral. In case you were marked for liquidation, the self-liquidation will also remove the liquidation mark and the associated deadline above.