There are a few amounts of products in the market that will clean the oven in to no time but I decided to have a go at cleaning it with more environment-friendly products. 
So, this is my list
                   - White vinegar  
                   - Lemon 
                   - Bicarbonate of soda 
For this way of cleaning, it will be needed:
- Lots of old cotton rags
- Gloves
- Stainless steel scouring pads
NOTE:  My oven is electric and is not a self cleaning, so if you are using this method to clean a gas oven, PLEASE make sure the pilot light is out and the gas has been turned off before starting cleaning.
     STEP 1 - Turn on the oven on the higher temperature possible.
     STEP 2 - Once the oven is hot, get a oven proof bowl (that way you can see if you need to add more water and vinegar as it goes along)      
                 - Pour boiling water 
                  - Handful of lemons
                  - 100ml of white vinegar  
    Put the dish with is contents inside of the hot oven on the bottom rack of the oven, close the oven door and leave it for 2 hours REMEMBERING to keep a eye on the level of the water and adding  more hot water or vinegar when necessary .