I use anything from large glass jars (the 1 gallon+ size) to clean water jugs, 2 liter soda bottles.. some people use buckets with heaters.. I don't, because the water drips in slowly and if the line is placed over the heater, it won't fluctuate the whole tank.
I add water conditioner and break up a piece of IAL into the container.
Place a portion of airline tubing into the water container that is long enough to go from the container to the tank (have to rig it to stay.. when using the water jugs I rubberband the tubing to the top, the open glass jug you can do the same with larger rubber band, or place something heavy on top.. for bucket (I don't use) but imagine can tape it down or use a clean rock or glass ornament and rubberband the tube to it.
Make sure the container is higher than the tank.. I have a heavy duty shelf running above my spawn tanks to hold the containers (among other items).
Cut the airline tubing anywhere you like - I cut it where I can access it easiest - and I personally just use air control valves
(some work better than others - I prefer the blue ones).
Place the air control valve on the top of the bottom portion of the tubing, then suck on the end of the top portion to get the water flowing, and then connect it to the control valve.. then adjust the valve to allow as little or as much water flow/drip as you want.
Similar to acclimating fragile fish to a new aquarium.. let gravity do the work. I place the airline tubing under the lid of the fry tank over the heater just to make sure there aren't any "cold" spots.
This way there is fresh water going into the tank for as long as you want, without it going in too quickly to harm the fry. Using just the drip system can take a while to fill up the tank.. so I use it along with placing in a bit of water every other day or so, basically siphon out a good amount of water, replace with some fresh and allow the drip system to add in the rest/more.