First of all you have to prepare the basic chain as shown. Now yarn over and the first three chains of the crotchet are always considered as the double crotchet. So you have to go to the fourth chain from the hook and then insert the hook. Now you have to drop the loop. So now you have three loops on the hook. Now yarn over and go through the first two loops and then again yarn over again and go through the last two loops. This is the double crotchet. Now finish this rope by using the same process now chain three loops in the hook and then turn over as shown. Now because these three chains are going to counter the first double crotchet, so the next one shall go into the next stitch. Once you know how to make these single and double crotchet you can make any design using crotchet because these are used to make most of the patterns. That' it.