For the wash I've found a video that said to first soak them 30 mins in cold water, then pour that away, rinse the pads.
Then another 6-12 to overnight in another bucket with cold water and they added a stain remover.
Like vanish in example.
But for those in the usa another product was used with a phalys napta bar (not sure how to spell I think it just sounded like that) which you rub on the stain before putting it back in the water.
The vinegar (white natural vinegar) is used to make the fabric softer. I tried this out myself on some fabric I hadn't used yet, I just let the fabric soak in the sink with water (if all is clean it can be just hot water no cold added) with a ... (not exactly measured you just pour a bit of the vinegar in the water) , which seemed to work. Although it makes sense to me to pull the fabric through the water at least a couple of times.
It is important to get all the stains out before you wash them, because blood doesn't get out of fabric when it gets heated.
You could use warm water, but definitely not that warm that it almost scalds you (stay within the range of 15-30 Celcius for that unless all the stains are removed).
Here is the video:
https://www.youtube.com/watch?v=82FjqcMvox0External ↗
As for the question weather they'd be seen through anything.. you'd have to look in a mirror.
If you have a problem like a yeast infection some mention using 3% hydrogen peroxide with it. (after cleaning them of course).