Do you mean fresh mackerel or smoked?
Removing bones from fresh mackerel is fairly straight forward if you are using fillets - just lay them flat on a board and cut a V shape right down the line of pin bones and lift it out - there won't be any more bones in the flesh. If using smoked mackerel fillets, the same applies, but it is a bit harder and you may have to waste a bit more of the fish. I often just cut the smoked fillets in half, lengthwise, cutting out the line of pin bones. It is a bit wasteful, but it is quick.
I have seen a video somewhere of cutting the flesh around the tail of a gutted, but not opened up, mackerel, cutting the head off behind the gills (diagonal cut) and, holding the fish down on a board, pulling the tail and the spine out, still attached to all the bones. Must try it next time I can get hold of some fresh mackerel.