when you get them more then likely they will be in a bag in a polly box, or they will just be in a polly box.
if its in a bag open the bag and let it tip out gently into the box. if theres loads of water remove some with a jug or something till the water is around 4" deep without stressing the ray out. if it looks to be stressed or not likeing it then just leave it alone.
get an airline tube put one end in your tank, tie a knott init to restrict the flow to a drip every second or so and place the other end in the box. let the pipe fill the box slowly allowing your water to mix with the rays water.
when there is twice or three times the depth you started with in say a couple of hours then stop the drip. leave the ray alone all the time in the box, put the lid on the box but leave a gap for the pipe and to allow a little light in so the ray can atleast see a little.
when its all done get a big net, pull the net tight so its like a tennis racket and put it under the ray and try to get the ray to move onto the net. when its on carefully lift it out the water and straight into the tank. if you lift and the ray flaps about dont lift it out till it settles abit or it will end up on the floor! which you do not want ever.
there are other ways, like a shallow large dish or something depending on ray size. DO NOT use the net like a net as the stinger will become tangled in the mech and it will cause a great deal of stress and youll have to cut the net, net left on the stinger can cause infections. trying to get it off is a nightmear.
leave the ray alone dont sit infront of the tank and leave the lights off for a couple of days. allow the ray to settle in with teh least amount of stress. do water changes the day before and dont add a ray to a tank that is new. it needs to be running for atleast 2 months fully cycled before you even think of putting a ray inthere. make sure the filters can handle the bio load and what fish you have in there, have extra which can be removed before the ray is added so the bio load stays the same in the tank.
this is how i do it and it has worked for me, others have different ways.
that is how i do it