Here's my recipe- may not work for everyone but I have had some success with it and it is quite forgiving.
50g Rye Flour
50g Strong Bread flour
125/150ml Water - Mix to consistency of thick double cream, different flours can take different amounts of water.
Once mixed keep warm 25/26 oC
keep covered but not airtight.
Try to use unbleached organic flour - at least to get started, once healthy and active I use own brand.
Use water that has been through a filter, if you don't have a filter sit a bowl of cold water out overnight with a squeeze of lemon juice in, this will help to de-chlorinate it a bit. Again once healthy & active normal tap water will be okay.
When I can start to see some activity (day 2/3ish), bubbles up the side of the bowl etc, I take half of the old mixture out and add in the same 100g flour/125ml water mix. I use it from about day 7.
I've messed with mine before adding a teaspoon of sugar or honey to speed up the process, and kept it in the fridge to slow it down too & have frozen it before and revived it. It is a bit of a labour of love!