An easy way to work out the volume in "ml" was to fill a liner with water then pour the contents into a cup to measure the volume and weight.
I could have measured the mould with a ruler but this was quicker and gave me an indication of approximate weight of resin needed to fill the volume of the mould, you also have to factor in the displacement of the item being encapsulated.
I estimated water would roughly be a similar density and weight to the resin.
Now you know the volume you need to follow the instructions for the resin you have purchased to find the correct ratio of resin to hardener.
I used Polycraft DSM Synolite Water Clear Casting Resin + MEKP Catalyst (1 to 2%), I believe it is a polyester resin the ratio of Catalyst to Resin was around 1%.
It was pretty hard to measure out the catalyst in such small amounts.
There are many varieties, which all require different ratios of resin to hardener.
So mixing it etc. is really down to the type you use.