The angel of Yahweh
Possible meanings are 1) Yahweh made himself look like an angel or 2) this was one of Yahweh's angels or 3) this was a special messenger from God (some scholars think it was Jesus). Since the phrase is not well understood, it is best to simply translate it as "The angel of Yahweh" using the normal word that you use for "angel." See the note about this phrase in Genesis 16:7.
a second time
The word "second" is the ordinal number for two. AT: "again" (See: rc://en/ta/man/translate/translate-ordinal)
Here the word "heaven" refers to the place where God lives.
said—this is Yahweh's declaration
"spoke this message from Yahweh" or "declared these words of Yahweh." This is a formal way of saying that the words that follow come directly from Yahweh.
by myself I have sworn
"I have promised and I am my witness." To swear means to use the name of something or someone as the basis or power on which the oath is made. There is nothing more powerful for Yahweh to swear by than himself.
you have done this thing
"you obeyed me"
have not withheld your son
"have not kept back your son." This can be stated in positive form. AT: "were willing to offer your son to me" (See: rc://en/ta/man/translate/figs-doublenegatives)
your son, your only son
It is implied that God knows that Abraham has another son, Ishmael. This emphasizes that Isaac is the son that God promised to give Abraham. See how you translated a similar phrase in Genesis 22:2. (See: rc://en/ta/man/translate/figs-explicit)
I will greatly multiply your descendants
"I will cause your descendants to increase again and again" or "I will cause your descendants to be very many"
as the stars of the heavens, and as the sand which is upon the seashore
God compared Abraham's descendants to the stars and the sand. Just as people cannot count the huge number of stars or the grains of sand, so there would be so many of Abraham's descendants that people would not be able to count them. AT: "beyond what you can count" (See: rc://en/ta/man/translate/figs-simile)
as the stars of the heavens
Here the word "heavens" refers to everything we see above the earth, including the sun, moon, and stars.
will possess the gate of their enemies
Here "gate" represents the whole city. To "possess the gate of their enemies" means destroying their enemies. AT: "will completely triumph over their enemies" (See: rc://en/ta/man/translate/figs-synecdoche and rc://en/ta/man/translate/figs-metonymy)