I'd say take the 3 from the very bottom and go up as far as you want with the 3.

Then do the 1 low. Then take the 2 and go from the area that is 1 up to midway up to the 3.

Then take the 1.5 and try to blend out the line between the 1 and 2.

If there is a line between the 2 and 3, it may not be that visible. If anything, try to do a rocking motion with the 2, or run the 3 over it again as best you can. Just don't take the 2 up higher into the 3 because you will defeat the purpose.

Then take the razor come and slowly try to blend in the top into the 3 area so that you can get a nice blend that you can blend in with gel.

That's the best advice I can offer for an at home cut in method. I can't guarantee the results because I'm not there watching you do it. But, based on the advice I've given and the knowledge I think you have, you should be able to pull it off just fine!

Good luck sir!