I’m not exactly sure where the two fuel filters are on the 85 300D. You should have the main filter very close to the power steering pump. It’s up high and should be easy to access. I believe on your car it’s aft of the steering (it’s left on my 87 SDL). It should have a bolt on the top center and look similar to a typical screw on oil filter. It may have a fuel line coming out of the top as well. I think the correct socket/wrench would be a 17mm. Before you get started pick up a small fuel container (and only use it for diesel). Purchase a gallon or so of diesel. After removing the old filter, position the new one and fill it with clean diesel prior to inserting the bolt. The remaining of the gallon of diesel goes in your tank. I would not store diesel. You may find a priming lever or pump handle on your engine near the fuel pump. If it has one, use it. My SDL does not. One of the lines leading into the main filter should be coming from the second filter you’ll need to change. This is the inline filter, or pre-filter coming from the tank. Be careful to install the new one in the proper direction! When you go to start, expect the engine to run for a second or two then die. It may take some cranking and a few re-starts to get it going. Most parts stores around here don’t carry the filters. I prefer ordering a hand full at a time online, along with some oil filters.