2 ideas:
1. I would use a guide of some sort at least. If it's mounted in a stationary base (rather than a 'portable' base like a router base) then you can run the wood against a fence of some sort. Doesn't have to be fancy; just put a piece of straight wood to the platform part of the stand, line up the bit, clamp the wood down to hold it in place, and then go from there. If you can set a stop depth, you should do that, too.
2. Use a straight-edge and an Xacto or other thin blade to score your straight lines as deep as you can. Then tape off the surrounding area to the best of your ability. Then, 'freehand'. But as you freehand, just come as close to that scored line as you can. In many cases, the rotation of the bit will send the last millimetre (between the bit and the scored line) flying. If it doesn't, use a small chisel later, with the scored line as a guide, to clean it up.
When I was inlaying my block inlays (all straight lines) I did a combination of using a hole-saw cutting attachment (used it as a mini router-base) and technique #2. Using the scoring technique, it's pretty easy to use a fine chisel (even if it's a construction-grade one... mine was like $8 or something) to clean out the bits that get left behind by not going right to that scored line. Even then, because it was my only major attempt at inlaying, I goofed one of the blocks up... I practiced on scrap first, but probably not quite enough. If this is your first inlay, I recommend putting in more practice time than I did.