You're doing BIP39 wrong... the process is explained quite clearly within the BIP39 documentation here:
https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki#generating-the-mnemonicYou don't START with 12(/15/18/21/24) words... you start with the necessary amount of ENTROPY... ie. a very large random number.
We refer to the initial entropy length as ENT. The allowed size of ENT is 128-256 bits.
First, an initial entropy of ENT bits is generated.
So, you can use dice or whatever other RNG process you prefer to create your 128-256 bits worth of ENT (ie. 128 coin flips)... then you calculate the checksum:
A checksum is generated by taking the first ENT / 32 bits of its SHA256 hash. This checksum is appended to the end of the initial entropy.
So, you SHA256 your ENT... then take the first bits of this hash and append it to the end...
The following table describes the relation between the initial entropy length (ENT), the checksum length (CS) and the length of the generated mnemonic sentence (MS) in words.
CS = ENT / 32
MS = (ENT + CS) / 11
| ENT | CS | ENT+CS | MS |
+-------+----+--------+------+
| 128 | 4 | 132 | 12 |
| 160 | 5 | 165 | 15 |
| 192 | 6 | 198 | 18 |
| 224 | 7 | 231 | 21 |
| 256 | 8 | 264 | 24 |
To get from this very large number to the actual mnemonic... you then convert your ENT+CHECKSUM into words by chopping it into 11bit chunks... each 11bit chunk represents a number from 0-2047... you use this to lookup the words in the 2048 word list.
For instance... say that you generated ENT+CHECKSUM (converted to binary) started out with the following binary sequence:
10010110111011011011100010110001011101010110.....
this would split into 11 bit chunks as:
10010110111 01101101110 00101100010 11101010110....
which converts to decimals as:
1207 878 354 1878....
Which converts to the words from
the BIP39 English Wordlist:
notice horse clump turn....
(note: you have to add 1... as the bit range values are 0-2047, but the wordlist as shown on github is indexed from 1-2048)
You can help me? How can I convert my 12 words that I chose myself, from a list of 2048 words, into a wallet?
If you want to choose your own (a really bad idea btw)... you will be able to choose 11... and then the 12th word you'll need to find through trial and error to find the one or two that generate the correct checksum.
Also, note that Electrum does NOT use BIP39 unless you select the "Options" button and check the "BIP39 Seed" option. It uses it's own custom (NON-BIP39) method for calculating seed checksums etc.
The last 7 bit of a 12 word seed are the checksum.
It's not... it's only 4 bits for a 12 word seed, as per the table above from
the BIP39 docs