Selling stuff online

I’ve been playing with the idea of selling some hand made stuff online. But I don’t know how to start.

 I’ve been reading about it but can’t really understand what I have to do. 

I work full time and I pay tax and insurance. 

So I’ll have to pay more tax but there’s also this £1000 trading allowance. 

It’s confusing. 

I understand that I would have to register as self employed and notify revenue about my income but I don’t know if I’m supposed to do it before I start or after I earn £1000 or when? 

I want to just put 1 or 2 thing online and see if anyone would buy it. 

If not I’ll abandon the idea. 

If someone buys it then I might sell more. 

But I don’t know if I have to inform tax office that I put one item on sale? And if it won’t sell? Notify them that it was a mistake?

Does someone know how to proceed?