there are many ways to start Tally Prime in Developer mode.
follow this simple process :
(You must have Tally Prime in your computer)

1. on your keyboard press Windows logo + R (Goto run command)
2. Right click on Tally Prime and goto properties and find shortcut tab.
3. Copy Target path (“C:\Program Files\TallyPrime\tally.exe”)
4. Paste in run command
5. Add /devmode in Last (“C:\Program Files\TallyPrime\tally.exe”/devmode)
6. Click on Ok

Now Tally Prime will Open in Developer Mode