Is the room where you are doing this totally neutral, where neither have been before? The area around the bonding space should be neutral to reduce problems of territorial behaviour. If they have been in this space before it can be neutralised using something like a 50:50 solution of water:vinegar
Is the 'bonding space' neutral? As above
What size is the space you are using? Ideally this should be around 8 square feet, the greater space you have the harder it is to separate them, this increases the risk of injury and fighting and the longer the bonding will take.
Inititally I would put nothing in to the pen for the first hour apart from some yummy hay and a small amount of food sprinkled.
If they start fighting pull them apart using a broom, hold apart for a slow count of 10 and then release. You may need to do this a few times. If they are constantly chsing and fighting do not pursue and consider other options.
If you do not feel confident doing the bonding it may be an idea to get someone more experienced to undertake this for you.