Take all the racks out of your oven, and put them on a couple of old towels in the bottom of your bath. No bath? Shower tray perhaps? And block the drain. Nothing to harm the surface as long as towels are put down to prevent any scratches. From the hot tap and maybe a kettle of boiled water pour enough water over the racks to cover and mix in a dishwasher tab or 2, or gel. Leave in the liquid overnight.
In a dish mix up some baking soda with a little water - enough to make a creamy paste. Use rubber gloves to make using your hands easier and then smooth the paste over the surfaces of the oven (avoiding heating elements etc.,) Wipe over the door last with the paste and then leave overnight.
In the morning scrub off the racks in the bath with a hard brush or sponge and, if necessary use something like Cif on any remaining stains. Rinse well and dry.
Wipe off with a cloth wrung out in hot water all the surfaces of the oven. Any nooks and crannies or difficult parts can be sprayed with some white vinegar - which will cause the baking powder to bubble and lift off stubborn marks. Clean and dry, clean and dry the door.
Elbow grease required in some cases but no chemicals, no worries about splashes or fumes and most of us have the ingredients to mix up for cleaning purposes.