Firstly, you will need to have a SPL compatible wallet.
An indefinite list of wallets can be found here: https://serum-academy.com/en/wallet-support/
You want to ensure the wallet you choose has the following supports:
Serum DEX support
ERC20 <> SPL conversions
One wallet that offers all this is the Sollet.io wallet: Sollet.io
So once you send your funds from an exchange or another one of your wallet to your SPL supported wallet you can instantly connect your wallet to the DEX or SWAP platforms via the chrome extensions.
It is strongly recommended that you make a safe and secure record of your wallets seed phases and store them safely.
A limit order is an order placed at an agreed price by the buyer and seller.
A market order is an order that is actioned at any given price. This type of order allows for slippage to occur and price may fluctuate.
A DEX works by connecting your personal wallet and sending funds to an intermediary account. Failing to settle your account, will have your funds stored temporarily in a wallet.
If you forget to remove your funds from the temporary account. Don’t stress, once you log back into the DEX your funds will be in that temporary account associated with your wallet.
It is important to settle your funds after each trade. Failing to do so will show a discrepancy in your wallet.
A complete guide to trading on the DEX can be found here: https://serum-academy.com/en/serum-dex/trade/