# But its inner parts and its legs he must wash with water

The person would do this before giving the pieces to the priests to place on the altar. 

# inner parts

This is the stomach and intestines.

# he must wash

Here "he" refers to the one making the offering.

# It will produce a sweet aroma for me

Yahweh being pleased with the sincere worshiper offering the sacrifice is spoken of as if Yahweh were pleased by the aroma of the burning sacrifice.

# an offering made to me by fire

"a burnt offering to me"