Sometimes it's hard to know where to insert your hook when you are joining a round with a slip stitch. In this example I will use ch-3 to demonstrate.
Use a safety pin or stitch marker to mark the ch-3.
If the ch-3 wasn't marked, you might have thought that you would join the round by inserting your hook where the arrow is pointing to the next stitch.
To join with a sl st, insert your hook into the ch-3, pick up 2 loops from the ch-3.
Pull the yarn through the ch-3 and then through the loop on the hook.
Finished. As you can see in the photo to the left, there is a little gap between the ch-3 and the next stitch. That is caused by the ch-3 being a skinnier stitch than the double crochet stitch.
If you need to make a double crochet increase in the same stitch, first ch-3, then yarn over and insert your hook where the arrow is pointing, yo, pull loop through st, yo, pull through 2 loops on hook, yo, pull through remaining 2 loops on hook.