A great question as many colorists are still stuck in the 80’s, using caps. Here’s the breakdown:
The technique or placement of the foil is the architecture of the highlight. This gives you the contrast and dimension of shape.

Horizontal will always produce a highlight that goes from side to side/across. Vertical will always produce a highlight that is chunkier and that caters to the depth/interior of the hair.

• The more foil, the more highlights.
• The larger the space between foils the more pieced the look.
• The more hair in each foil the heavier the contrast.
• The less hair in each foil the more natural/subtler the look.
• The amount of foil used, amount of hair in each foil & the space between determines the look.

You may either slice or weave, it’s your choice. But make sure you have neat, clean sections and subsections. Make sure foil is secured using proper folding and that your slice/weave is not too far from the scalp. At the same time, make sure you do not apply product at the very top of the foil, leave a minimal space. Then fold your foil slightly higher than your placement point.

Don't make the color on the foils to thick... Less is more and when doing the closest to the roots, tap it on. The foils will create heat, which will expand the color forward.

And we’ve saved the best of all for last....foil causes no discomfort to the client.