This is a problem if you use standard leader knots for mono-to-mono joints, although some anglers add rig glue to secure these. Here’s a mono-to-braid leader knot based on the Bimini Loop.
First, tie a large overhand double loop in you braid (the width of the hand is about right, or just larger). Make a lasso out of the braid loop and pass it over your mainline. Take the end of the mainline and wrap it six times back up the outside of the loop and then insert it inside the braid loop and wrap three or four times around the inside until the end is out of the loop. Tease very carefully tight and the job is done once you have trimmed the end of the knot.
Other options include the Grinner, but you may require a minimum of five turns.