How do you damp proof a concrete floor before screeding?

By the sounds of it you have Marley tiles down and bitumen. Be careful as some of those types of tiles contain asbestos. The best solution is to uplift the tiles and dispose of accordingly, then use a specialist screed such as F-Ball Stopgap 1200 straight over the bitumen. Then once the screed is dry use a liquid DPM such as F-Ball Stopgap F77. The tiler can then latex or screed over the DPM. However the tiler should apply a Primer before screeding such as StopGap P141. This will allow the screed to bond to the dpm.

You should apply a paintable damp proof membrane (DPM) before the screed. You can purchase this from most DIY stores.

Damp proof sheeting black plastic should do it. Wrap up the ends make sure you have insulation standing up against all external walls to stop cold bridge. The plastic should be above the screed at the edges

Take up the existing tiles Use something like ball f77 liquid dpm But first id get a floorlayer with a damp metre to take readings first