To keep crayfish in your pond you first need to create hiding places,you can use rocks to build this or use plastic tubes,the ones used for plumbing or drainage are good they need to be 20-30cm long and have a diameter of about 10cm,you can make a cluster of 2-5 tubes and weigh them down to keep them in place,be sure the materials you use do not leach chemicals into the water.
You will need to have atleast one hiding place for each crayfish as they will hurt eachother if they are unable to hide.
Crayfish will try to escape from your pond until they are adapted to the water,so you will have to put them in a cage in your pond for one to two weeks before you let them roam free in the pond.
Crayfish are sensitive to copper,lead and zinc so be sure to use ungalvanized corosion resistant fittings in your pond.
The water needs to have a PH close to 7,5 but as long as the PH is stabile the crayfish will be fine so do not worry if the PH deviate a bit from this,the water needs to contain some calcium for the crayfish to replace thir shell,crayfish are able to store calcium in their body for later use so you do not need to add large amounts of calcium to the water.
Crayfish are mostly acctive at night so do not worry if you do not see them during the day,if you use a flashlight at night you will see them searching for food often close to the surface.
Be sure the pond is well cycled before you add crayfish,plants or fish into the pond.