First of all, you need to gain fluency in both the English language, as well as in American Sign Language. For that purpose, you can join any ASL school or academy, and start by taking beginner ASL courses. Do not expect to learn the language quickly, as the process of learning ASL is same as for any other language.
Keep learning ASL until you start thinking in ASL.
It means that you should get expertise in the language to an extent that you perceive things as per the ASL language.
Observe ASL interpreters and talk to them about how they learned the skill of speaking English language fluently, while translating it to ASL. You may get some good tips which you can use to make your learning process efficient.
The syntax and sentence structures vary from language to language. Same is the case with ASL. Therefore, you should not try to use the same sentence structures for ASL as you use for English language.
You can start with translating some basic sentences first, like “my white cat is sitting near the table”. Here, you first need to decide what the emphasis is on in this sentence. In ASL, the order of the sentence is based on the word with more emphasis.
Here in this example, if the emphasis is on “My” then you need to sign "my" "white" "cat" "window".
If you want to emphasise the colour of the cat, due to the presence of two or more cats present near the window, then you can sign "white" "cat" "mine" "window".
Similarly, you should sign "window" "white" "cat" "mine", if you want to emphasise cat’s location.