DIY Glycolysis

Go on to implement the conversion step.

Convert the Dihydroxyacetone phosphate into Glyceraldehyde

Good Choice.

Remember that lactate has a carboxyl group on carbon 1. One way to make a carboxyl group is to oxidise an aldehyde group. So your choice of working with glyceraldehyde phosphate is a good one since it has a aldehyde group at carbon 1.