How do you do multi tim video

I cant figure out what the trick to this is hope some one can explain..

I am trying to edit a video but when I try and do muti trim it wont let me now will any of the edits I do take afect I select the input/start marker ans the out/end mark but the clip is still its full length

Q1: how do I make the changes stick?

and when ever I try and mark another set it erases what I have marked

Q2: how do I mark more than one section of a clip?

It sounds like you are setting in and out points in the Clip monitor. After that, to "keep the changes" you have to click and hold the monitor and drag it to the timeline.