Self tapping thread inserts are one of the best ways to create thread inserts in 3D printed parts (see Tom's video here).
Simple accurate and strong, these inserts can be screwed directly into a part.
I have included threaded cutouts in STEP file format that you can import into your project and use to create the appropriate space for the insert.
Includes cut-outs for M4, M5, M6, M8 and M10 at various lengths, that will accommodate bolts from M2 up to M6.
The inserts I use, available in a variety of sizes can be found here and here.
A test block is included for you to check tolerances.

How to use
1) Model your part using the cutout STEP file to create the position for the thread insert.
2) Use a bolt with washer inside the insert for insertion
3) I use a little epoxy glue to secure within the part
4) Take care getting the thread started to avoid crossthreading
5) Use the bolt and washer to get the insert level with the print surface