Rules of liquid yeast work.
1. Use wort of sg 1.030 -1.040. for 1.040 use 100gms DME per litre. Aeration is important, you are aiming for healthy yeast growth, not beer production.
2. Be as aseptic as possible (as close to sterile as possible.) Take great care with very small amounts of yeast and wort transfer.
3. The smaller the quantity of yeast, the smaller the starter.
4. The older the yeast, the smaller the starter.
5. Once you have your small wort actively fementing, step up in scales of 10, for instance, 100ml is stepped up to 1,000 ml.
6. For ales you need an active starter of volume 5% of your wort. So if your batch size is 23 litres, you need a starter of 1.175 litres, most people aim for at least a litre. Lagers are slower to multiply and ferment, they need double, or 10% of your wort size. If making a stronger og brew, consider using fresh yeast slurry from a previous brew.
Aim to have your final step close to the volume needed.
7. Consider using a good quality yeast nutrient blend in your starters (not just plain diammonium phosphate, something that has lots of trace elements). Healthy parents have healthy children. Make sure you use the correct does, too much micronutrients can be harmful to your yeast.
8. Every time you work with yeast, sniff it and if possible, pour some into a glass and taste it. This way, you have an understanding as to whether the flavour/aroma is funky infected or funky hard working yeast.