General principles...
Place clips side-by-side on a single track. Place text on the next higher track; it will automatically be superimposed. Remember that higher tracks take precedence over lower, so if a full-frame image is on track 2 it will cover what's on track one. If it's made smaller, track one will be seen in the background (picture-in-picture).
When inserting a clip from the (top-left) bin between two existing clips, be sure that the scrubber (red line) is precisely at the cut point between them.
Use these buttons to ensure that this is the case... 
A good way of doing this is to right-click the bin clip, then PLACE CLIP ON SEQUENCE AT CURSOR. When a clip is inserted at a cut point, all following clips move to the right.
If a clip is inserted within another clip, it will split that clip. Not doing this correctly can cause strange clip placement. This can also happen if a clip is dragged on the timeline. It could end up where it's not wanted.
Switch to Storyboard mode to see clips on track one. In that mode it's easy to spot unwanted snippets and gaps. 
To close gaps, right-click and CLOSE GAP.
Clips must be long enough to accommodate the transitions between them. Shorten the transition if necessary. Otherwise it may not work or do odd things.
When laying down audio, LOCK the video. The track cannot be edited when locked and it won't be moved.
Anytime you make an error, immediately press <ctrl-Z> (same as EDIT | UNDO) to backtrack, step-by-step.
Save AS often. Do not press the SAVE button in the toolbar. Instead, click the chevron at its right and Save AS, with that unique # or name.
