Getting a Sudanese visa is actually quite easy.
It is probably easiest to apply in the country you are in before Sudan. Allow a few days for this.
In my case the embassy in Addis Ababa had run out of stickers, so was not issuing visas.
I then tried in Dublin; applying is easy, but the wait for approval from Khartoum can, and did, take some weeks, too long for me.
So back I went to the embassy in Addis. filled in my form, paid my $68 and returned the same afternoon to be given my visa.
You do need proof af accommodation for at least one night; in Addis all they really wanted was an address. You can find somewhere on AirBnB or some other website. Look carefully at the cancellation conditions if your travel plans are not set in stone.
On entry to the country, you have to register with Alien Registration Dept within 3 days. This can be done at the border – in the same office as immigration – when coming from Ethiopia. It cost S£640 (US $1 = SDG75) and was straightforward requiring a copy of passport, visa and a photo.
When I needed to return to Sudan after my accident, I contacted the embassy in Dublin and asked if my previous application was still valid. It was and all I had to do was pitch up with my passport and get the visa, for €60.
This time I entered by air and did the alien registration at the airport. This is much more convenient that going to the immigration head office, but more expensive (SDG 1,100) It is done at a travel agent kiosk located between the two exit doors. (Contact No: 0912 508 106)