On mobile devices, the easiest way is to install the Slack app from either the Play Store or the Apple Store, whichever is appropriate.
On devices like laptops or desktop PCs, just start your preferred web-browser and go to welshspeakingpractice.slack.com
This “WSP” workspace is open to everyone, but you need to have an invitation link which should have been sent to you via email. And if you are on a structured course, you should also gain access to a dedicated workspace nicknamed 6/6. In either case you can email admin@saysomethingin.com asking for the invite links again. (Maybe check your email spam folder?)
Tagging @Deborah-SSi for further assistance.