put this elsewhere but here goes.
want to make a 'glass mould shape same as outside of a Weber kettle BBQ lid, how do I approach this.

Do you want to make a half sphere essentially - do you have the object to duplicate?

Lets say you want to make exactly a Weber kettle lid. You would smear the whole thing with mould release wax, dry, then repeat say 3 or 4 times. Kiwi etc shoe polish will do OK to avoid $50 spend on the real stuff.
Then lay up tons of fibreglass over that, and support with a wood structure of some sort. Think about like papier mache over the balloon like we all did in primary school.

Rip it off when cured. NOT as easy as it sounds, depends on size and flexibility of the mould or if you can destroy the plug (original item)
Of course also the shape needs to be such that the mould can slide off. You can;t mould from a complex shape without all sorts of specialness that's not doable for a first timer.

Now you have a mould.

Same procedure - lots of waxing, then lay glass in it. Rip it out when cured. You now have a fibreglass Weber lid (dunno why lol, but its a reasonable example of simple first time moulding project)

thanks Mark, I only need the first part. If I use a weber lid, cover the outside, then all that will be seen is the inside of what has been made.

What Mark said is 100 percent.
But do you need a female mould? Once you have done your mould release wax you can fibreglss over.
In terms of changing shap you can get moldable clay to give bigger radius or fill in temperature gauge.
For preparing if you can wrap in plastic first (I've used kitchen film or packaging film) to fully protect the surface and masking tape to trim the edge to and then release wax.
You can sandwich in between fiberglass to add strength.
If you use polyester you can have gel coat finish.
All depends on what you want. Good luck