DHA drops are good for the odd tan. they come in dropper bottles and you need to use multiples of the whole dropper (if that makes sense). 20 drops is one dropper full.
50% DHA is the best.
The maths is a bit mind boggling, but here are the instructions for Siennasol 50% Booster drops.
Professional Spray Tan Solutions
10 drops per 100ml of tan will increase the DHA content of the tan by 1%, we generally recommend you add 20 drops per 100ml of tan & test it, if it is not dark enough then add a further 20 drops on next application.
Daily Moisturisers
Add 2 drops per squirt, again if not dark enough add another. We generally assume you will use 20 drops over the whole body, so if using with a facial cream, add just 1 drop.
The amount of drops used depends on how dark you wish to go & what the drops are added to. With a little experimentation, you will soon know how many drops to add to your favourite tans or moisturisers. You can layer it up if you need to go darker.
Here are my pictures. The leg is Siennasol Extreme Extra 20% DHA. The arm is Siennasol Extreme Extra wash off. Both are the really dark tans and probably too dark for most people (good for dancers though).
Siennasol Extreme Extra 20% DHA.
Siennasol Extreme Extra wash off