You can cut a piece of cardboard, stick a little hole in it for the stem and push it through.
Put it over the water so the stem is hanging in the cup.
This can take time, but it will root.
When i water clone it takes between 10 - 20 days mostly.
But i have had some that just didnt want to root which took almost 2 months.
Use a dark cup so no light gets into where the roots are gonna be.
You can also use a humidity dome "you can use an old bottle which you cut"
This will help.
But i must also say similar to timfbmx & UnEmploymentDude.
If its possible i would get out and get some Clonex or other good rooting gel, powder etc
and some jiffy pots or rockwool. I used to water clone but since i used
Clonex i never gone back. 7-10 days and you got roots poping out everywhere.
Best of luck either way you go.
Happy growing,
_forbidden_