To fit a new platform, first check that the replacement looks suitable. A new unit is easily marked and subsequently too soiled to return for exchange.
If the pivot guard needs to be rotated to suit a contrate wheel to one side, carefully remove the balance cock, complete with hairspring, balance wheel etc. Put these very carefully to one side. Now very carefully remove the plate holding the lever and escape wheel in place. With tweezers, lift out the lever and escape wheel. To rotate the guard, hold the empty platform in one hand and grip the guard in smooth jaw pliers. You can protect the jaws from marking the surface with a little piece of card. Now very slowly rotate the guard to suit the clock. If you turn it sharply it may shear off.
To fit the platform, it is safer to remove the balance as described above. You will need the escape wheel in place to establish the depth needed from the contrate wheel. A good way to mark the hole positions is to fit some sharpened taper pins in the existing holes. Fit the pins so that only 1mm or so protrudes. Place the platform in the desired position and move slightly from side to side. The sharpened pins will scratch the underside of the platform where the holes need to be drilled. Always drill the holes slightly larger than the screws need. This will enable the platform to be finally adjusted. The distance from the escape wheel pinion to the contrate wheel is critical, and this final adjustment will have a noticeable effect. Platform fixing washers cover up these holes and ensure that the special screws used for platform fixing hold the platform firmly.
Re-assemble the platform, being very careful to fit the pivots into their jewels before tightening. Gently ease each pivot around it's jewel hole until it pops in. When fe-fitting the balance wheel ensure that the jewel on the balance wheel is between the forks of the lever. When the pivots of the balance wheel are properly in their jewels, tighten up the screws. Never tighten up screws on a platform if the pivots or staffs are not properly seated in their jewels. Just the weight of the balance cock itself is enough to hold the balance in place. Additional pressure will simply cause damage. The leverage effect of a screw will bend a pivot of this size without you even knowing.