Ask your HR department for a quote for notional service purchase (this will set out both how much you can buy and the cost per year of notional service); get an AVC transfer value; make the purchase.
You should consider whether you want to transfer all of the value of your AVCs into notional service. For example, you might be more interested in maximising your (currently tax-free) lump sum than your ongoing pension...in the event you would not be able to afford to fund full service by retirement date, you could max. your lump sum without having to fund all of a full ongoing pension if you hold onto some AVCs. It's complex and you should consider some genuinely independent advice (which you obviously won't get from the AVC provider who would benefit from you keeping more of your AVC than might really suit you).