Making the Beracha
- One should sit while reciting Birkat HaMazon. Some say one should also sit for Al HaMichya. It’s preferable to sit for Boreh Nefashot, but in cases of need one may say it standing. 
- One shouldn’t make a Bracha while one’s hands are dirty rather one should wipe one hands. 
If one Mistakenly ate Without a Beracha
- If one already swallowed his food but plans to continue eating, he should recite a bracha before doing so. He should not recite a beracha if he doesn't plan to continue eating.  In such a situation, if possible, one should say a bracha and eat a little more. 
- If someone ate food without reciting the bracha beforehand, but realized before swallowing, the proper procedure will depend on which type of food he put in his mouth. If it is a food that will become disgusting if spit out, he should put it to the side of his mouth and recite a bracha. If it will not be disgusting to spit it out, he should spit it out and recite a bracha before putting it back into his mouth. 
- If one drank a liquid but forgot to make a beracha before it, and remembered before swallowing it, he should swallow the liquid and not say a beracha rishona on what he already drank.  One can, however, think the beracha in his mind while the liquid is still in his mouth.  Others advise spitting it out as long as it is not a pressing situation. 
- Even if one omitted the bracha rishona, he should still recite a bracha achrona if he ate or drank a sufficient amount.
Preparing the food
- The food must be in front of the one making the Bracha, otherwise, the Bracha doesn’t have what to be effective upon and one would need to be a new Bracha.  If the food that’s brought afterwards is of the same kind as the one that the bracha was made upon or it has the same Bracha as the food that we made a Bracha on originally. 
Text of the bracha
- For every Bracha it’s very important to say the words as they were established by the men of the Great Assembly. Words that are crucial to the Bracha and if one misses one of them, the Bracha doesn’t count: Baruch, either Hashem or Elokenu, Melech, and HaOlam.  If the Bracha has a conclusion of Baruch Atta Hashem… then the name of Hashem is also crucial. 
- S”A 183:10 writes that one must sit while saying Birkat HaMazon and some say one should it while saying Al HaMichya. So agree the achronim including Yalkut Yosef 183:7. See Mishna Brurah 183:35 who writes that this is according to all opinions. Kaf HaChaim 183:51 and Ben Ish Chai Chukat 4 write that it’s proper to say Boreh Nefashot seated. VeZot HaBracha (pg 8) writes that if there’s a need one may be lenient and say it standing.
- Kaf Hachaim 158:53 Shaar haTzion 181:32, VeZot HaBracha (pg 8)
- Berachot 51a compares one who eats without a bracha to one who ate garlic, causing him to give off an offensive odor. Should he eat more garlic and increase the odor? One who transgressed by eating without a bracha certainly should not transgress more! This is brought down as the halacha in Rambam 4:2, Tur S”A. 167:8 and Vezot Habracha pg. 88.
- Berachot 51a quotes the opinion of Ravina that even if you are finished eating you can recite a bracha. He brings a proof from the fact that a convert or baal keri (machloket Rashi and Tosfot there) can say a bracha on his tevila in the mikveh after the tevila. The gemara rejects his opinion and distinguishes between the two because the convert or baal keri could not have said the bracha before but the person who took a drink could have. Rashba Berachot 50b s.v. “vihatanya” quotes the Raavad who paskins like Ravina that even if one finished eating he may recite a bracha (see Maamar Mordechai 172:2 who writes that the Rashba’s version of the Raavad contradicts the Raavad’s own note on Rambam Berachot 8:12). However, Shaar Hatziyun 172:5 writes that this opinion of the Raavad is a daat yachid which we do not accept. Therefore, S”A. 167:8 rules that if he is not going to continue eating he should not recite a bracha. Yalkut Yosef Berachot pg. 118 and Aruch Hashulchan 167:18 agree.
- Magen Avraham 167:21 writes that, if possible, one should say a bracha and eat a little more in order to satisfy the opinion of the Raavad quoted in the Rashba. M.B 167:49, Aruch Hashulchan 167:18, and Yalkut Yosef Berachot pg. 118 agree. Rabbi Meir Mazuz (Ish Matzliach 167: footnote 7) says based on the Beit Yosef that this is not necessary. Shaar Hatziyun 167:45 and Shaare Tesvhua 206:2 add that if done immediately this could help retroactively to correct having eaten without a bracha. Therefore, Or Litzion 2:46:12 and Vezot Habracha pg. 88 say that in a situation where you are saying a bracha on what you are going to eat, you should have in mind to cover what you ate already.
- Chazon Ovadia Berachot page 68. Berachot 50b quotes three beraitot regarding what do in a situation when one ate before reciting a bracha. These beraitot appear to contradict each other. One says to swallow, one says to spit out, and the third says to put the food to the side of one’s mouth. The gemara explains that one should swallow if it is a drink, you should spit the food out if it won’t be disgusting to eat it after that, and you should put it to the side if it will be disgusting to spit out. This is quoted as halacha in Rambam Berachot 8:12 and S”A. 172:1-2. Vezot Habracha pg. 88, Chazon Ovadia pg. 68. M”B 172:7 explains that the reason that one must spit out foods that will not be disgusting is because the gemara teaches based on a pasuk in Tehillim that when reciting a bracha one’s mouth must be void of any object or food, so that it is “filled” only with God's praise.
- Chazon Ovadia Berachot page 68, Shulchan Aruch 172:1. The Rosh (Brachot 7:33) quotes the Raavad who explains that when the gemara says one should “swallow the water,” it means that he should swallow the water and then say a bracha rishona. He explains that since he remembered before he swallowed, he can swallow and then say a bracha. This situation is different than remembering that one did not say a bracha until after he has completely finished eating because in this case he remembered while the water was still in his mouth. He notes, however, that if one has more water, he should spit out the water, say a bracha, and drink the water that he has. According to some, this is the opinion of the Rambam in Berachot 8:12 as well. However, see Chazon Ovadia Berachot pg. 69 who proves otherwise. Biur Halacha 172:1 s.v. “Viayno” writes that ideally one should be strict for the Raavad to spit out the water if he has more. Vezot Habracha pg. 88 agrees. Although Rama 172:1 follows the Rosh, the S”A 172:1, M”B 172:5, and Eliya Rabba 172:1 say one should not recite a bracha rishona if he is not going to drink more (see a list of other poskim who hold this way in the Shaar Hatziyun 672:5).
- Chazon Ovadia Berachot page 70-71, Ben Ish Chai Matot 14, Kaf Hachayim 172:1, Pri Megadim M.Z. 172:2 and A.A. 172:1, Ketzot Hashulchan 55:9.
- M”B 172:2
- M”B 171:3
- S”A 206:5
- S”A 206:5
- S”A 214:1, Mishna Brurah 214:4
- Mishna Brurah 214:5