Forestay tension is achieved by tensioning the backstay. On most boats the amount of backstay tension you need to apply is less than the resultant forestay tension because the forestay makes a more acute angle with the mast than does the backstay.
However, on many catamarans the backstay makes a more acute angle than the forestay, as can be seen on this Prout sail plan. This means that for every kilogram of tension needed in the forestay you have to crank on more backstay tension. So, be careful.
A Loos tension gauge is a handy tool for setting up standing rigging.