Both, SSL buying and renewal process are same. So you have to follow the basic steps that you already know to enable SSL certificate on your website. Let's Encrypt has auto renewal process for domain validated certs but if you are looking for anything else like OV or EV, then you will have to go for other paid provider.
According to CAB forum guidelines, Certificate authority cannot issue DV certificate more than 39 months validity and EV certificate for more than 27 months validity.
So you can get DV/BV certificate for maximum for 3 years and EV certificate with maximum 2 year validity because most CAs prefer to issue certificate on yearly base.