It is not hard, just timing. Tough to get a big batch to be real close to each other.
There is a You Tube video I believe but pretty simply, just pour some hot plastic into the sprue of the closed mold and immediately shoot hot plastic into the mold. This will give you close to what you have with those culprits.
Doesn't work to good with 2 cavity molds.
For hand pour, it is pretty tough and the timing is crucial. Pour a layer, wait and then pour a thin layer of not to hot plastic so it stays intact and does not run to the sides of the mold. Then quickly pour the top portion. Not this easy but those are the steps but not the timing. Not the best picture here but you get the idea..
You can also use hand pouring and injection to achive some type of core. Pour the bottom, then add the core color and close mold and shoot... red core frog...