Time and Bar Line

Look at this thread and at this one .

HTH,

M.

The "Right" way.
The most "technically correct" way of splitting a measure is to create (insert after the measure to be split) a new measure and change the duration of the first one to 3/4 and the second to 1/4. If you had a large piece and were counting measures carefully then you might also want to set the second one as "irregular" so that it wouldn't add to the measure count.

The "Easy" way
For your purposes, though, where it does not seem that you are counting measures in a big orchestral score and you are not placing repeats at the bar split and you just want a nice legible piece of music, then the easiest way is to enter all your notes, click on the note before which you wish to place the split and then double-click on your chosen barline (double, single, dotted) from the Bar lines palette. So in your example, you click on the quaver (the first of your "two small notes") then you go over to the Bar line palette, pick the one you want (the double barline with both lines the same thickness) and double-click on it.

If you want to (and it is a matter of preference rather than being a musical necessity these days) you can then make the first measure of the whole piece 1/4 and the last 3/4 to keep the appearance the same as the original.

So why is there a "Right way" to do it when the Easy way appears to work? Well, the Easy way looks OK but it causes problems with playback in MuseScore when it comes to putting repeats at the split or sengos (the squiggly - S sign) etc. and then MuseScore needs "real" measures and barlines rather than just one that looks OK.