How do I use Stimpaks and RadAways on Dwellers in the Wasteland?

Solution 1:

There is no way to force them to use them, they will just use them on their own until their stock you give them when you send them out goes away.

Nothing (that is in the game right now) can cause them to die in a single attack that they would not be able to heal from so no worries there. When they get up to the radiation amount a RadAway would heal then they will use one, then their health is low enough for a Stimpack to fill it up, they will use it.

A good ratio is 2 to 1 from Stimpacks to Radaway. I generally will send out my guys with 20 Stim Packs and 10 Rad Aways though they will often still return with a RadAway or two on them. I will also do 15 and 10 if I am low. Your residents will never need RadAway unless you run low on water so as long as that is ok do not fear sending more with your guys in the wastes.

Solution 2:

As far as I know, you don't. They decide when to use them.