I use a tool bit ground into a kind of short cut-off tool shape. I don't have a digital camera or I would post a photo. I'll try to describe it the best I can.
1. I take a regular high speed steel bit, and grind a front relief and square-up the end at same time.
2. Grind both sides of the bit giving it side relief and at the same time making it thinner at the back than the front. Kind of like the shape of a dovetail joint in wood working.
the length of the side grind should be a little longer than the depth of your groove.
The width at front should bee a little narrower than the bottom of the groove.
3. Then I put a little top grind on it. Just like a sweeping hollow grind, to get chips flowing while cutting.
Then do like you said plunge straight in to the depth, angle compound and cut each side to correct width as shown in Machinery's Handbook.
Hope this makes sense to you.