I failed a three batches of salt bridges before managing to have one stable made.

• Mix 200 ml water, 5 g of agar and 75g of salt in a cooking pan.
• Bring it to a boil while mixing regularly.
• Let it boil for a few minutes (not too much, otherwise you will loose your water)
• Pour the boiling mix in appropriate containers
  • Reused glass jars (with an opening having a size close to the widest diameter of the pot)
  • Glass petri dishes
• Let them cool down a bit
• Place them in the fridge for a few hours

• Use hot water from a kettle and mix everything in a cold pot
  • This induces a very uneven mix, where salt and agar are not well dissolved
• Let it sit in the pot beore pouring it on top of the substrate
  • Warm liquid may harm the microbes in the substrate
  • Most of the agar and salt will sediment at the bottom of the pot, and your mix will not become a gelatin
• Do not forget that the fridge step is important !
• I highly suspect gelatin to be inefficient for this process. My successful batch was made from agar-agar from a chemical provider. I think other sources may be fine, but I have not managed to use them properly yet.