John Gill Commentary Numbers 18:26

"Moreover thou shalt speak unto the Levites, and say unto them, When ye take of the children of Israel the tithe which I have given you from them for your inheritance, then ye shall offer up a heave-offering of it for Jehovah, a tithe of the tithe." — Numbers 18:26 (ASV)

Thus speak unto the Levites, and say unto them
Aaron was to tell them what a provision would be made for them on account of their service, but Moses was to inform them what part Aaron was to have out of it:

when you take of the children of Israel the tithes which I have given
you from them for your inheritance ;
which was done at the time of their harvest, and of the ingathering of the fruits of the earth:

then you shall offer up an heave offering of it for the Lord ;
in token of gratitude for the liberal provision he had made for the maintenance of them:

[even] a tenth [part] of the tithe ;
a tenth part out of the tenth of the produce of the land of Israel.