As others have said, mark your lines all the way round. Get a comfortable grip on the saw, point your first finger down the length of the saw (as opposed to gripping with it). Line your saw up to one side of your line (the waste side), shoulder in line with the saw and the cut line.
I use the thumb on my grip hand to line the saw blade for the initial cut. Start with small gentle strokes then build up using long smooth strokes finishing the cut with small strokes again. Don’t force the saw, it should be a smooth action.
Its all about practice and muscle memory…… I’ve been a joiner practically all my life, everything is about practice until it becomes second nature.
I love Jap pull saws….. A slightly different action to a western saw but I find them much more accurate and intuitive to use.
For planing end grain you’ll need a block plane, preferably a low angle…… It needs to be razor sharp, sharp enough to shave with. But planing end grain is not easy and you can easily make a mess of things. Best to get your first cut perfect.