Firstly, never use your stove with cracked or missing glass.
If possible remove the stove door if it can be simply lifted upwards and off, otherwise leave it in place. Lay the door flat on an old blanket or piece of carpet as this will make removing and fitting the glass so much easier and safer. However, in our experience most doors will need to stay in place. If possible wear a protective face mask to avoid breathing in any soot and debris if the door is very dirty and always wear protective gloves when handling the glass. At this stage it is also worth thinking about replacing any worn door rope seal at the same time as the stove glass.
Always use penetrating oil on the glass clip or frame screws and allow plenty of time for this to work before you attempt to carefully remove them with the appropriate correctly sized philips or slot screw driver or allen key. It is important that these screws are not damaged as finding satisfactory replacements for them on an old stove could prove difficult. If the stove is over six months old then always replace the glass gasket at the same time as the glass. Clean the inside of the door frame and remove any remnants of the old glass gasket. The new gasket will provide greater resilience against the spring clip and allow sufficient space for the glass and cast iron door to expand without the glass cracking. On some stove models the manufacturers have deliberately left off the gasket running along the top of the frame and on some models they may even have left a small obvious gap at the top. It's important therefore to exactly mirror where the old gasket sits as altering this may adversely affect the stove's performance or Defra status.
If you've had to leave the door in place then it may be a good idea, once the old glass and screws have been removed, to place an old cushion or similar underneath the door before attempting to fit the new glass as this will reduce the possibility of damage to the glass should you let it slip when handling it (it can be fiddly and it's happened before!). It's always helpful to have another person help you hold the glass in place while you fit the clips. Always fit the lower clips and screws first as this will allow the glass to safely rest on them without it sliding out of place while you work your way around the screws – but always keep the glass vertical as any pressure from the glass pressing up against a screw at angle could crack the glass. Do not over-tighten the screws ensuring only that the glass does not rattle or move. The new gasket should take up any slack and provide the appropriate gas-tight seal. It is advisable not to relight the stove for 24 hours to allow any adhesive gasket tape or rope glue time to dry naturally.
NB. Damaged or cracked glass cannot be recycled and should be made safe and labelled as 'heat resistant glass' and placed in a non-recyclable bin. Alternatively please seek advice from your local council.
Please note that some models of wood burning stoves have more than one piece of glass in the stove and therefore it is important to check that the piece specified in the listing is the piece required.