I use some old glass containers that once held scented candles that I got from someone on Freecycle.
First measure out how much water you need. Simply fill each container to the required depth with water and pour into a measuring jug. I needed just short of a pint.
Boil sufficient water and dissolve the gelatine following the packet instructions. I generally like a firm gel so I use rather more than the minimum recommended. Add a couple of teaspoons of salt to the mix to avoid the gel going mouldy.
Whilst the water is boiling place add the essential oils to the glass jars. The more you use the stronger the scent. It doesn't matter which oils you use if you just want room scenters - just chose ones with a pleasant scent and maybe some of the cheaper ones. If you are using a couple of containers you can of course use different oils in each of them
I often have other objectives so I will use a blend depending on what I want to achieve. This batch contains a blend of patchouli, rose attar, nutmeg, clove and a couple of others. You are going to want 30-40 drops of oil per vessel. If you want a coloured gel then you can also add a few drops of food colouring. I don't generally bother as I like the natural amber colour but each to his//her own!
Once the gelatine has dissolved simply pour the mixture into the jars, stir well with a toothpick ( use a different one for each jar if you have a different blend in each one) and leave in a cool place to set.
I find they last about 4-6 weeks before needing replacing but that will depend on the location and the atmosphere.
