I am reminded of the joke - there are a 100 ways to do this and all of them end badly.
Depending on the state you are in (or country), you are required to use special handling including getting a permit
to do this. Presuming none of that applies, then here is the start of what you need to start:
1. If you can flush it with a degreaser to start, that will help. However, I cannot think of a place in the US where the outcome of that is legal
to dump anywhere.
2. oil-dry absorbents - these are chemicals you can pour into the tank to dry it out. That should not be confused with "dried out." You hit a spark, even with absorbents, and its way 99 of a good start to a bad day.
3. oil-dry pads - put these under it. No matter what you do, something is going to spill and the CEO is going to smell it for a month and you will hear about it for a year. Put the pads where they can absorb anything that drops.
Now it may seem logical to fill the tank with absorbers and vacuum it back out. Rethink that. Oil
laden material thrown into a high speed electric arc
(way 98 to get a bad day started). I have seen guys repair fuel tanks
by putting dry ice in them. The CO2 is supposed to dampen the possibility of a spark. The theory is good, but bad execution.. (way 97)
Now I am sure someone will say you can just cut the tank because diesel fuel does not burn easily. And - they are correct. However, if you happen to vaporize some while you are cutting, well - that would be way 96...
So, given the legal
, EPA and explosive ramifications, this is one you may want to leave to professionals (or take out the motor).