Reaction Kinetics-reaction mechanism! Watch
Each step can only involve two particles colliding.
Therefore two of the particles must collide in step one, then a product of that 1st step must collide with the so-far unused (one of the initial three) particles.
1x ClO2 and 1x F2 are involved in the RDS - since they are both in the rate equation and since they are both first order (this determines that there must be 1x of both).
The easiest thing to do (which is rarely the correct thing to do, but this doesn't matter at A level) is to simply combine the two particles into one.
In this case, just make Cl2O4 OR ClO2F2 in your 1st step.
I'll leave you to work out your answer.