Be aware of different meanings that come with different ASL signs whether you mean "last night" as in "yesterday" and "last night" as in the "the last thing that will not appear or happen again." Or something that continues in time.
Here is how to sign "last" in American Sign Language whether it's the night before or the thing that will not appear again.
Meaning: Coming after all others in time or order; final.
Pronunciation (sign description): Horizontal dominant "i" hand hovers a little above the horizontal non-dominant "i" hand. The tip of the pinkie moves down flicking the tip of the passive (non-dominant) pinkie.
For example, if signing LAST WEEKEND in this context, it means the final one, not the previous weekend. No more weekends after this last one.
Context/usage examples: "going to the beach for the last week of June", "this is the last book we have in stock".
ASL words have their own one-word or compound signs for some English two words with "last" in them. For example, "last year", "last minute", etc.
Meaning: On the last occasion before the present; previously.
Pronunciation (sign description): Dominant flat hand with palm facing the shoulder moves toward the dominant shoulder.
For example in this context, when saying "last weekend" in English as in the previous weekend, it's signed (glossed) as PAST WEEKEND in ASL, not LAST WEEKEND which means the final weekend.
Context/usage example: "slept late last night", "she got married last July", "the last person to know".
Meaning: (of a process, activity, or state of things) continue for a specified period of time.
Pronunciation (sign description): Two-handed "10" hands with both thumbs in contact and palms down move forward simultaneously.
Usage example: "the performance art lasted 25 minutes", "nothing lasts forever".
For at last as in "at the end of a period of time", see FINALLY.
First 100 words.
As you feel more comfortable with the first few hundreds of ASL signs, progress further with your vocabulary and learn signing more.