ONE: CHOOSE LETTERS OR SOUNDS AND CREATE AN ALPHABET
This is just what it says. If you go to the section Alphabetic and Phonetic and scroll down to 'exercise', it will give you a step-by-step on how to do this.
TWO: MAKE RULES FOR YOUR LANGUAGE
What word order do you want? How do you want your adverbs to work? Should adjectives come before or after nouns? Etc. If you want to create something really simple, you'll either change your first language so it is simple or keep the first language exactly as it is and only create new words.
Using English as an example, I could say I want 'I' to be nen, and then I want possessive to be the particle dou; I don't want 'my' or 'mine', instead I want nen dou.
THREE: MAKE RULES FOR SUBSTITUTION
The sounds or letters you picked will together be known as 'letters'.
This bit is where you are going to make rules for how you will create your new words. The most commonly used method has been flipping words backwards: Ho ym Suratrat! Or flipping sentences backwards: Suratrat ym ho! (Used in Harry Potter and the Chamber of Secrets on the Mirror of Erised.)
Most conlangers wouldn't do that though. For a language game you can switch letters around, and this is very easy. All you do is create a chart for which letters in your first language will become the letters in your second language. This is actually a kind of cypher as well.
I usually split it up by vowels and consonants, because, if you interchange them, you'll probably end up with too many consonants and words will be difficult to say. This would not be a probably for languages such as Japanese, as Japanese letters are either one vowel, a consonant and a vowel, or a consonant and two vowels, -- the exception being n, which is the only single consonant in the whole alphabet.
- A --> E
- E --> I
- I --> O
- O --> U
- U --> A
- B --> D
- C --> S or K
- D --> B
- F --> G
- G --> H
- H --> J
- J --> K
- K --> K
- L --> L
- M --> N
- N --> M
- P --> R
- R --> P
- S --> S
- T --> T
- V --> V
- W --> U
- X --> KS
- Y --> I
- Z --> S
That's the substitution part done. You can use sounds not native to your first language too, but I'll keep it fairly simple here.
If I were to just use that system to create words, I could say 'Hello! How are you?' becomes Jillu! Juu epi iua?
But how about I change it a bit? How about when two consonants are next to each other (mm, nn, pp, ll; etc.), the second consonant becomes an a. And j can never start a word, so any j at the beginning of the word (which was originally an h) will become a v. So, then it comes Vilao! Vuu epi iua?
If you had originally decided you wanted an SOV language, you would have it Vuu iua epi? Then maybe you would want the particle nar between two words that end/begin with a vowel to make things easier to say clearly: Vuu iua nar epi?
FOUR: CREATE WORDS
This is where you create words using the rules you make. It's easiest if you translate sentences to make sure everything sounds right and works out okay and agrees.
Since you're probably creating so many words, it's easiest if you make different categories you will translate in: So, one time you will translate words for greetings, then words for family, then words for numbers and colours, etc.