I used a ready-made mat earlier this year for a bigger project, which was really easy. I was trying to cover a small shed this time so thought I'd try doing it myself - I used some sedum plug plants and some really stony soil.
I've just done this on a small shed using a mixture of sedum and moss. Total cost was less than ?40.I used pond liner to waterproof and a drainage layer I bought from amazon. I have spare left of both which you're welcome to have if you want to give it a try.