After losing all my power tools and not having a table saw Allah has gave me an idea how to cut small parts with a hand saw and to get a great result. I am not handsaw master like that guy on unplugged woodworking on youtube but alhamdulillah I have managed to get a straight cut.
Step 1: Placing the Ruler Part
Take one bigger part that is square cut and put clamps over it and the part that you want to cut. Be careful when placing ruler part that you put in on the line of the sawing cause if you put it 1 mm to the line so you can see the line your part will be shorter for 1 mm because of saw thickness. I advice you to draw two lines, one cutting line and another 1 mm apart. So you can align the ruler part directly over the line and other line as a guide for a straight angle. Check everything with a square ruler and start cutting slowly. Don't make too much force, just lean at the ruler part and saw easily.
Step 2: Result
You will get a straight cut inshaellah without cutting mistakes and losing too much time on the vise. It is even better if you have aluminum or a steel square part to use it as a ruler just watch at your saw if it is sensitive.