An acrostic is essentially the opposite of an acronym. It begins with letters and makes words that start with those letters. Ideally, the words combine into a sensible sentence or phrase because words are much easier to remember than sequences of letters. Acrostics are popular ways to remember order.
For example, many automobile license plates begin with 3 random letters that can be made into an easy-to-remember acrostic. DHP can "Darned Highway Patrol" and LDT might be "Look Don't Touch" or "Let's Drive Tonight." A more challenging example arose when we were in the Kimberly Mountain area of Outback Australia. Another tourist told me about a good book that described the early days when Australia was a British colony. The author's name was Durack. Having nothing with which to write down her name, I devised the acrostic, "Down under, Read About Colonial Kimberly" as one way to recall her name later.
There are no constraints in creating mnemonics. You can combine the acronym and acrostic methods. The first four (inner) planets, in order from the Sun, are Mercury, Venus, Earth, and Mars. The acronym MVEM is not very helpful. The other (outer) planets, again in order, are Jupiter, Saturn, Uranus, Neptune, and Pluto. That acronym JSUNP is also not very helpful. Now consider the acrostic: "My very easy mnemonic. Just spin us nine planets." You can memorize two sentences and use the first letter of each word to help you think of planets.