(I am assuming the “spackle” is US for filler.)
Have a firm base for filler does help, as you need to be able to push on the filler to get it well into the side of the holes. However as it is not the end of the world if the filler breaks when you try to send it, I would try the easy option first.
For larger holes I have built up with layers of filler passed into the side of the hole then left to dry for a day before doing the next layer. This sounds like a real pain, however if you have lots of holes and you don’t need them fixed today, you can just walk round your house each day making each hole a bit smaller. The key is to push the filler well into the side of the hole so it grips well, trying not to have it coming to far in front of the wall, so as to reduce the amount of sending you will need to do. (No professional would do this, as it need to many visits, but you have not travel costs to your own house)
Pushing newspaper or a sponge through the hole so it expands behind to provide a base for the first layer of filler has also worked for me in the past.
However if the hole is more than about 2cm across, you should be fixing a hard backing of some sort.