With the gun in a fired position and apart, assembly can occassionally be awkward, the barrels will go on easily but perhaps the forend will not and you are worried about breaking something. If you assemble a fired gun and carefully align the forend and close it you will perhaps to use a little more pressure than normal to get it to seat. Once the forend is locked in position when you open the gun the gun will cock as normal.
If the above does not work with your detachable lock gun or if you're afraid of breaking something follow this method.
In order to assemble the gun the lockwork will need re-cocking meaning the hammers being put back in the 'ready to fire position', with a detachable lock gun first open the cover plate as above and rest on the edge of a table or similar, something that will not scratch the steel.
We use a rounded piece of copper but a thin piece of hardwood, plastic or brass will also work. You do not want to use steel on steel as it will scratch. Put the end of your tool in the position shown on the foot of the cocking dog.