My novice advice, having just worked through this process on two locks and double set triggers.
1. Position as others have said, with the "V" often the adjuster screw position right under sear bar.
2. Get trigger plate inlaid to flush, I do this with lock out
3. With lock back in, and sear bar with in letting black, put trigger in. For sure, both trigger plates need to be filed down so they do not contact sear at this point.
4. Then test through cocking motion, likely sear may be a tiny bit lower in half cock, maybe full cock. If so, be sure main trigger is filed down for a bit of clearance here. If a single trigger you might now be done. But, when pulling set trigger, it will raise the main until it clicks in place. So file more on the main until it will not hit sear when setting. At this point the main is done and will have a bit more free play than a single trigger would need.
5. Now all attention is to the set trigger. Its spring will be holding it up higher than the level of the main trigger. It must not contact the sear in this position. If it touches too much, you will not be able to get sear to catch in the half or full cock notches. Furthermore, it should have a tiny amount of free play in this unset position. Test with inlet black, file, repeat as needed. Keep testing through the cocking and setting and firing process. (Put a wood block in jaws for this, instead of a flint.)
6. When it works as it should, polish both trigger bars to a slick smooth top edge. You know it is working as it should when you can put the trigger in and pull to full cock and you then remove the trigger seeing no inlet black on either trigger bar, then put trigger back in and set (set trigger moves down away from sear when setting) then slow and easy pull main trigger. It should pull with no free play until set is released. Set trigger should then spring up with enough force from its spring to fire. Then pull lock and verify that set trigger bar only has inlet black on it (from springing up to fire), the main should have no black because you pulled it slow and easy stopping before its free play for single firing is used up. Lastly, clean trigger bars, cock and fire single. Then check, in this case only the main should have inlet black from sear contact.
Experienced builders please.correct if I am wrong.