To make sure the floor screeding is the strength that it is needed, the sand and cement should be measured accurately to make sure the proportions are correct.
Also, the method used for mixing is important. You should know what proportions are needed when mixing by hand and when mixing with a cement mixer.
If mixing by hand, make sure the dry material is mixed in thoroughly. Clean water should be used instead of dirty water. The mix should not end up being too wet either. The amount of water used needs to appropriate for the amount of other materials that are in the mix.
When using a cement mixer, make sure that a small amount of water is added to the drum before adding the cement and the sand. Adding the cement may cause it to stick to the drum and adding too much water can aggect the strength of the mix.