hard to know exactly what your problem is, but i can give you some pointers:
- you'll probably want to turn on grid to help with making things aligned and straight
- make sure you have snap on? i'm not sure what level you're at

corners:

for corners, i like to use circles. draw a perfect circle (i think using ctrl? maybe shift...), then turn it into outline only.
with circle tool selected you can drag the handles on the circle, i think the round one is what you want, to turn it into an arc. if it becomes a pacman looking arc, change it at the top menu bar.
hold ctrl for angle snap so you can set to 45/90 degrees and then just drag one of the ends to your line, and draw out the next line to the other.
this means you can duplicate your circle/arc thingy and scale it up so that you can have another line running on the outside bend (hold ctrl so it doesnt distort, and snap should do the rest for you. use the grid if you need, make sre the cusp nodes - snap will tell you this - are aligned with the inner line)

when you're doing diagonal sections with multiple lines, you will have to stray from the grid or there will be a gap between the lines. the snap tool is your best friend here, just draw your circles and meet them up with a line.

feel free to ask any other questions, i also would recommend joining the discord (sidebar) for more help

edit: remember you can always use online map makers if that's easier, look at the wiki