How can I design a VLSM network?

You have to learn: IPv4 addressing, classes, Masks, subnetting and then supernetting. After those you will be able to design any VLSM you want.

You should start with the largest number of hosts network you have then you move to the smal ones. And you should choose the first near block size to the number of your hosts, for example if you need24 hosts then you can go with32 block size which has the /27 prefix and if you need13 hosts you should go with a block size of16 which /28 prefix, of course you can go with a bigger block size but it will be a waste of ip address unless you expect a future grows for you network. you can use a helper table to help you design your network.

the following image is taken form :

CCNA: cisco certified network associate studey guide

the implentation of VLSM is little hard, you compute the host  each subnetwork before you implemnet vlsm. you must go1st to many host going to the few host until you reach of them. you use any classes of ip thats better for security. vlsm is the best ip implementation for network security.