Follow the directions included! Read the directions 1,2 or more times before cutting anything because seresto collars are an investment. Lay out the 2 collars together on your counter before cutting. Make SURE the secure away collar is adjusted perfectly and allow room for the seresto underneath before measuring out your seresto to fit the INSIDE of it. You might want to consider allowing a little extra room than you usually do for your dogs collar to keep the seresto from directly touching your dogs skin. Just follow the directions carefully and you’ll be fine. The reason I say all of this is because I’ve been using seresto collars on my Pittie for several years now and when I got this set up done and put it around Bronx’s neck, within an hour he was shaking, anxious, continuously swallowing and several other symptoms. The collar was not too tight according to the 2 finger rule. But, it was in fact too tight as far as the seresto chemical reaction scenario. Fortunately I remembered the very first seresto I bought breaking my dogs neck out in a rash overnight because it was too close to his skin. So I assumed the reaction he was experiencing was due to this same mistake. It really scared us and we removed the collar immediately. It took a few hours for Bronx to seem like the symptoms were subsiding. Then my thoughts went to “crap, I just wasted $68” bc I had cut the seresto to the current adjustment of this new dog collar. I thought this over for a bit and 5 days later(the weekend) we decided to try again. This time, we adjusted this new collar out to be larger than we normally would adjust it. We used the same seresto although it was now almost an inch too short for what the recommendations are for combining these 2 collars but, it STAYS! It still worked for our dog. The seresto doesn’t try to come out from under the secure away so we were lucky in that and now that it’s not too close to Bronx’s skin, it’s all good! Love the idea, works great. Just think it all through before you cut anything and you should be fine.