SushiSwap's liquidity pools allow anyone to provide liquidity via https://exchange.sushi.com/#/pool When they do so they will receive SLP tokens (Sushiswap Liquidity Provider tokens). For example, if a user deposited $SUSHI and $ETH into a pool they would receive SUSHI-ETH SLP tokens. These tokens represent a proportional share of the pooled assets, allowing a user to reclaim their funds at any point. Every time a user trades between $SUSHI and $ETH a 0.3% fee is taken on the trade. 0.25% of that trade goes back to the LP pool. If previously there were 100 SLP tokens representing 100 ETH and 100 SUSHI each token would be worth 1 ETH & 1 SRM. If one user trade 10 ETH for 10 SUSHI, and another traded 10 SUSHI for 10 ETH, then there would now be 100.025 ETH and 100.025 SUSHI. This means each LP token would be worth 1.00025 ETH and 1.00025 SUSHI now when it is withdrawn.
You need to provide tokens in a 1:1 ratio to the liquidity pool. This means that if you are adding to, say, a SUSHI-ETH pool, and wish to provide 2 ETH worth of liquidity, you would need to convert approx 1 ETH to SUSHI tokens first via our Swap.
If the pool you wish to provide liquidity to does not exist, you can create it of course! Just provide the tokens and off you go. As the first liquidity provider, you set the initial exchange ratio (price) if one of the tokens in the pair does not exist yet on SushiSwap. This often quickly corrects itself through arbitrage and by more liquidity providers adding to the pool.