If you are a shareholder of the company as on the record date announced in the rights issue scheme, then you will automatically receive the REs you’re eligible for in your demat account.
REs are listed on the stock exchange on the day the rights issue opens and can be traded on the exchange for about a week before they are delisted permanently.
You can even buy them from the secondary market after the issue's open date. You will not be able to transact them intraday as they are settled on a trade-to-trade basis.
If you purchase an RE on Monday, you will receive it in your demat account on Tuesday and will be able to sell it from Wednesday onwards.
In other words, you will need to wait for two trading days (T+2) to be able to sell REs bought on the exchange.