There isn't really any DIY involved
It's dead simple - Just use up to two grams of these Chempak compound fertilisers per five litres of water (that's 0.4 grams of fertiliser per litre of water) and adjust the pH to suit.
They each cost around £7 to £10 per box (for 800 grams) and if diluted as above one box will make 2,000 litres of solution, or more if used in smaller amounts.
I never go above 0.3 g/l (3 ml/litre), because that seems to suit my plants.
I use all three versions -
- #2, High Nitrogen (25-15-15), during the growth phase
- #4, High Potash (15-15-30), during flowering
- #8, Low Nitrogen (12.5-25-25), during the final two weeks or so
They all contain the following:
- Boron (B ): 0.020%
- Copper (Cu): 0.010
- Iron (Fe): 0.200%
- Manganese (Mn): 0.020%
- Molybdenum (Mo): 0.002%
- Zinc (Zn): 0.050%
- Magnesium (Mg): Unspecified but mentioned ('also contains')
As an alternative, there's Phostrogen (14-10-27 [3.5-2.5-6.75 after dilution]) which can be used all the way through the grow and at a slightly greater concentration of up to 0.5 grams/litre (or 5ml/litre if diluted to a 100g/litre concentrate first).
It contains:
- Copper (Cu): 0.010%
- Iron (Fe): 0.04%
- Magnesium Oxide (MgO): 2.5% (Mg 1.5%)
- Manganese (Mn):0.02%
- Molybdenum (Mo): 0.0016%
- Sulphur Trioxide (SO3): 7.5% (S 3%)
- Zinc (Zn): 0.0055%
Good luck!
ETA: There's also a 'Balanced' version of the Chempak (#3) which would probably work as an all-in-one, but I don't know its N-P-K ratios.