If I wear lashes on my lower lashline, I place them underneath the lashes, and close the gap between the natural and fake with eyeliner and eyeshadow really well.
I think the placement it the most important, I cut them shorter than my top lashline fake lashes also.
False lashes on the lower lid are usually smaller and more spaced apart if you want a natrual look. The best placement is below the eyelash hairs and from the middle of the eye (iris ) towards the ends. I like to use the transparent duo and get as close to the actual lashes as possible. After that I use mascara on all lashes and the result is lovely. Obviously if you need a touch up of any liner you should do that before you apply mascara.
You can also use the individual hair eyelashes and apply three or four on the bottom lash line -- these can be intertwined in the middle of the actual lash hairs.