There seems to be a lot of confusion about when to use what linking word in sentences. My students ask me constantly when to use 'but' and 'however'. My simple answer is 'however' is more formal than 'but'. For a more in detailed explanation of the difference here's where you should look, it's a link to another blog that has a post on linkers, too. It may help clear the confusion between 'but' and 'however'. http://madteachergraham2.blogspot.de/2013/02/linkers-for-contrasts.html There's only one way to be proficient in using these, sometimes, not so little words and that's to practice them. There are lots of things you can do to practice them.
Here's a link to a slide show that helps explain how to use contrast linkers in conversations.
http://www.slideshare.net/kesbu/contrast-linkers-17226445 There's also a really good explanation of the contrast linkers here: https://www.youtube.com/watch?v=nX8N9RiGCZg
Here's a link to some online quizzes that can be helpful for practicing. http://www.usingenglish.com/quizzes/89.html
If your students are having trouble distinguishing the difference between 'because' and 'because of', here's an online practice they could try. http://www.usingenglish.com/quizzes/220.html
One thing that I realize as a teacher and former student is that there sometimes aren't enough hours in the day to do everything you want to do in class and that students need homework assignments that are short and practical. Here's one that the students can do online http://writeenglish.net/usinglinkers1-1.php Students can check the answers themselves and see their mistakes so asking questions during the next class is a little easier. ELTbase also has a good number of worksheets that teachers can print out and use in class, or students can print them out and do them themselves. http://www.eltbase.com/vtr_refs.php?id=148
Here are cool vocab games to play. http://teflgeek.net/2014/03/19/six-great-vocab-games/
I really hope some of these links help you prepare for class and you can make it fun for the students, too. Good Luck!