Build an outer and inner form out of pieces of scrap wood or find plastic rectangular planters of two different sizes and use those instead . You will need a bigger and smaller container – the difference between the two is the size your candleholder will be . To ensure the candle comes out of the mould once it has solidified , duct tape the interior to seal off the wood , and do the same with the outside of the smaller form – this is if you are creating moulds from wood . Apply a generous amount of cooking oil to every surface the concrete will touch . This will allow the concrete to release from the mould . Mix your concrete as per the instructions on the package , then pour it into the form you ’ ve greased and prepped . Make sure it gets into every crack , crevice , and corner . Once it ’ s totally full , flatten and level the top .